First off, spells are your best friend.  Levelling up a mage champion early while judiciously saving up mana can really help in tough fights.  I've always played as an empire with death worship so I have access to a bunch of debuffs that take the bite out critters and AI troops alike.  Casting "wither" on a group you're going to attack helps quite a bit even before the start of battle.  When in a tough fight I generally move everybody back for a few turns until my sovereign can cast "infection" (spreads all negative spell effects to everyone else in the enemy group), followed by "blindness" (reduces enemy accuracy), then "curse" (reduces enemy armor to 0), and finally "graveseal" (makes all hits on enemies critical hits).  If all four spells stick, everyone in the monster group will have trouble hitting you and you'll have a much easier time eliminating them because of their nonexistent armor and constant susceptibility to criticals.  That spell chain usually requires five turns (infection has 2 turn casting time) so if the monsters are quick i usually sacrifice someone to slow them down (that early starting spearman or militia are usually the first to go).  Something to keep in mind when casting "infection" is to cast it on a unit that has low resistance, isn't likely to die right away (or else they won't have a chance to infect everyone else), and won't kill too many of your troops by being left alive until they infect everyone else.

Actually building a few early troops helps quite a bit too.  My custom faction has access to one handed spears, and my early leather clad spearmen with earliest shields (with health, defense, and increased defense when defending traits) can take a significantly higher beating than the spearmen and militia you get at the start.

Save up any cool scrolls that you've found for the tough fights.  There's one that I don't remember the name of but it drains 3hp from EVERY enemy on the map for each level of the champion using it.  With a level 10 champion you can take off 30hp from every monster regardless their defense.  A tough fight against a fire elemental, ignuses, and a bunch of fire shrills gets a lot easier when you can kill all but the fire elemental on the first turn.

Also, early on don't be reluctant to put more than one champion in an army for a tough battle.  I generally avoid doing so because you end up loosing some experience (50% less for every champion but the first), but if a fight is really crucial it is well worth considering.

The ophidians (magic resistant snake things) are particularly tough because you can't use any magic on them and they have pretty decent defense.  It'll be a tough fight anyway you cut it.  Maybe early spearmen can do some damage to them since spears ignore a portion of defense.

Stay away from banshees if you don't have any elemental damage weapons or spells (they're immune to normal damage), and stay away from obsidian golems if your troops can't hit them hard enough to bypass their tough defense and survive a few rounds getting squished into pancakes (they're immune to magic so spells won't help).

Some other critters you have no choice but leave alone at the beginning of the game.  Drakes, dragons, tougher elementals, and that poison staff guy with an army of stalkers and najas are really tough to take on without good armor, weapons and better spells and are better left alone.  Don't walk by them with weak units to avoid setting them off near your territory OR do walk by them with weak units and try to lead them off in the AIs direction while a champion pillages their goodie hut and your pioneer puts down a city.

I agree with pretty much what everyone else said. At the start it's a balance between being quite aggressive with your main stack to level up your heroes and not being so aggressive that everyone dies. I imagine most people have a story about the time they attacked Butchermen and got absolutely annihilated. It normally makes sense to avoid groups which are stronger than yours. Even if you can beat them with use of spells and good tactics you'll probably lose units, and you want to avoid losses unless there's no other option. It normally makes sense to attack monsters which are the same threat level, but be a little bit careful; make sure you understand any special abilities they have; as previously noted: you cannot cast spells on ophidians; you need magic damage to hurt banshees; you want to be a threat level above Butchermen before attacking them and preferably with zero units that the butchermen can kill in their first turn.

I'm a bit wary of units which are the same threat level as me but have higher hitpoints. It may be fine but I want to be sure I have a plan how I'm going to beat them. That plan will generally involve spells; Death magic has good curses, Life magic has Heal. Both sides can access good summons (Skeletons/ Wraith/ Lightbringer). Air elementals are powerful. Casting Fireball with a high initiative mage and/or with -1 turn casting time can make battles against ranged troops a lot easier. Understand what the weaknesses of particular monsters are; Air Shrills and Fire Shrills are ranged units, so if you can kill them from range quickly with your own troops and spells they are a lot less dangerous. Darklings/ Ruffians and other low attack troops are a walkover if you cast Wither or have decent armour. Bears can kill units with Maul, but they're a lot less dangerous if you can reduce their accuracy by casting Blind. Wolves are a lot less dangerous if your troops have spears and can't be counter-attacked. High armour units like Earth Shrills are a lot less dangerous if you can cast Curse.

Assuming your strategy relies on casting tactical spells then save up some mana before attacking monsters. Depending on what you want to cast and what it costs, 50 mana is probably enough for a fairly hard fight. Even if you think the fight will be easy, it normally makes sense to have ten or 20 mana so you can cast a Haste or Curse or whatever and make the battle easier, meaning you take less damage.

If you're not strong enough to take on a particular group of monsters, ignore it. I believe the world generation tends to place weaker monsters near faction starting positions, so in most circumstances you should have have space to expand without taking on impossible battles. I quite often leave a Strong or Deadly stack until quite late in the game. The only thing you have to be aware of is that you don't want to get too close to the monsters, and you may need to have a plan to defend any close-by cities if mobile monsters are spawned. Boosting the garrison is a possibility, as is making sure you have a stack within three turns move so you can cast Freeze/ Tremor and then get back.

In terms of just general expansion strategy, I always make my second hero a Commander who can build roads, and I build roads wherever I go. That way, if I find a good site my Pioneer can get there very quickly. Sovereign's Call is an excellent spell for "good" factions, it massively increases your growth rate, which means you can train pioneers much more often. If you have a city which is approaching its maximum population then you might as well get it training as many Pioneers as you can actually find a use for. If you have units which cost zero maintenance then make maximum use of them, and either lower your taxation rate (increasing production and research) or if you can generate a decent gold surplus on a low taxation rate then use the gold to Rush buildings.

Sometimes you can find yourself cramped by strong monsters and AI factions. This makes the game harder, but not impossible. Concentrate on building up your cities as much as you can, build outposts to get as many resources as you can, build a small but high quality army. If the AI is building troops with strong armour then make sure you're building troops which can damage them, which means magic staff units, Heart of Fire, rings and amulets. (If you can cast Mass Curse this is much less of an issue.) At some point you will either be strong enough to attack the AI or the AI will declare war on you anyway. If the AI attacks you before you were ready it's not necessarily the end of the world. You get the benefit of strategic spells that can only be cast in your territory, so remember to use them. You also get the benefit of city garrisons if the AI attacks your cities. Early in the game the AI normally attacks with two or three stacks, and if you can kill them then it will take the AI a long time to build new ones. So if you have to fight a defensive war, don't panic, and if you can beat the initial wave of AI attackers you will probably be able to immediately go on the offensive and take the AI's cities.

immune to magic snake things is obsidians, they're really tough until you have weapons with decent amount of attackpower, preferably at least boar spears upgrade from spears. Alot of stuff can die to boar spears. I even brought down a god with boar spears and an assassin

And Obsidian golem... i once got attacked by them early game where I only had spears but plenty of mana for heal spam.. took me crazy amount of turns in a single battle to bring one of them down with five spearmen and a sovereign attacking it

And then, for those times where early obsidian snakes roam around and attack you.. remember that you can get swarm bonus by having tons of soldiers surround the monster and attack it repeatedly even weak weapons will do mediocre damage. Just make sure to take soldiers that's running kinda low on health off the monster cuz they will get killed if obisidians doesn't miss with their powerful bites.

Yes, I had my starting village wiped out by butchermen once. x.x

Then my starting undead army got wiped out by.. mites :3.

After that humiliating defeat by mites, I don't underestimate anything anymore.

Try to recognize that blacksmithing technology is basically a benchmark to get in order for you to start expanding against more powerful monsters.

Stack either dodge-related trait or as above said, underdog. Rage is surprisingly good too.

i recommend simply playing as a beast lord.  Having direct control of those beasts will let you understand the mechanics of the game faster.  Youll have a better understanding of what to attack, and what not to attack.

have fun.