You know one of the best creatures to tame with Beastmaster Sov Perk?  Hoarder Spiders!  I copped two big hits from the spider (I started Tame before the first hit) and was reduced to 1 or 2 hitpoints, then my Tame was ready and I was successful!  (Note the first time I fought it I got defeated, but the Hoarder Spider's attack killed all its little buddies, so the next time I took it on there were no other spiders.)  It has one of the best specials in the game (Daze - which almost always makes all enemies lose their turn due to the extremely high Spell Mastery of the Hoarder), a high defense for a non-human unit/monster, and very high hitpoints.  Just watch out for the +50% splash damage on your own units - any unit next to the unit attacked gets 50% damage, whether they are friend or enemy.  That is how splash damage works - is not the size of the attacking units that matters - any unit apart from the splash damage creature itself that is next to the attacked creature takes the 50% (or whatever the splash damage percentage is) damage.

I kept on taming creatures from then onwards, into the Swamp Wildland, and lost a few along the way, and with the loss of three Nagas and about all my remaining 200 mana (or maybe more) including the Shrink spell, I defeated Morian, the Ruin of Summer.  I managed to use my Hoarder for a number of big hits (Morian regenerates 8 HP every one of his turns, so you have to keep doing a lot more damage than that) and retreated the Hoarder when it would have died with one more hit.  I think the fact that the Hoarder didn't kill me but left me with 1-2HP when my Tame spell was ready made all the difference.  For a long time, I was the highest rating side on the map, with just the one big town and all the hoarded (yes bad pun ) beasts I tamed.  I finally cleared out the swamp after finding the last pickup location to clear.  And I did all this without any reloading or crashes.

This is my Sovereign at the moment.  My Civ was Krax blood, Kingdom, Master Scouts, Enchanters (for Arcane Monolith and the Spellbooks) and Light Plate armor.  I like it how you can still survive and win the game with a minimal number of cities!

I think the least bad weakness you can choose for your sov is Weakness to Magic - as you can get resist items and when you level up you get more Magic Resist to offset the starting penalty.

Btw: is there any other way to clear swamp apart from Arcane Monoliths and Outposts?

This is my custom Sov created in 0.915: