Basically what I was saying, was to increase max spell points to 150 ASAP (via tech extensions) while you build any Spell Point producing buildings to fill that gauge. Hopefully you can time it so that you have already teched the spell point expansion (to 150 spell points) by the time your spell point gauge starts to fill up (approach 100) so that no spell points are wasted w/o having to buy a weaker spell.
Then, having your 150 spell points, your Fire book, and your Fire shard, with your ONE city because your saving your 10 essence for that Summon Fire Elemental spell which costs 10 mana.
Then, after you get the spell, start using "summon fire elemental", win battles with it, gain exp, gain more essence so that you can cast it multiple times per turn ...
And then attack a city, and throw one fire elemental at it (at a time) until that city is captured. Early game it will probably only take one, but you never know, so you personally should probably stay some distance away in case your first fire elemental is a bit overwhelmed ... but as long as it causes significant casualties to the enemy, summon another one and send it that way too. Also, if its possible to have MORE than one fire elemental summoned at the same time, then go to town 
Anyways, to get your FIRST fire elemental spell, it requires NO civics tech (or change in city size) NO warfare tech, NO diplomacy tech ... all it requires is 3-5 magic techs and a few buildings. Of course, after you have the magic techs in place, you can start focusing on civic tech to try and get a bigger city for more Spell Point buildings. Furthermore, once your revived land has begun to spread, you can build another city which will give you more spell points buildings ... although if it costs too many materials to build a second city, then skip it ... unless you have enough room for max spell points buildings + some materials producing buildings. In that case, then build more materials ... however it might be better to INSTEAD focus on Food + Huts so that you can build more magic buildings in the one city you already have.
So really, to get Fire Elemental ... you only need a Size 1 settlement (probably with a single study and magical academy), level 1, and you can get your spell much faster than a non-specialized player.
The two real challenges to this strategy are ... finding a Fire Shard before you place your first city
and ... balancing Research and Spell Points so that you research 150 max spell points around the same time you GET 100 spell points initially, for maximized speed.