[Makinus]

The first thing you must be aware is that playing agains the AI is a very different game than playing agains human players and tactics that are good against humans may be useless against the AI and vice-versa.

In the Thunderstrike case it is a good tactic against human players because humans rarely recruit chaff units (low hp, low protection, low price units) as they are completely useless mid-late game, so humans tend to have smaller/higher quality armies than the AI, that recruits lots of chaff. Against the AI the small number of units that Thunderstrike kills is pratically irrelevant, while against humans those same losses could cause a rout and win for you the battle.

For fighting AI hordes you must look at spells/tactics that do a small amount of damage to a large number of units, as the typical AI army will have a large number of low hp units.

In early game you should look at spells like Shockwave, however you must find a way to protect better your mages that will be closer to the enemy units... there are several ways in how to do that, either with buffs or itens... i like to use vine shields for close-in protection as they also help significantly against arrows, even giving 2 vine shields to more costly units...

Mid-game you should look at spells like Blade Wind and late-game your spells should decimate AI hordes easily as the AI simply cannot compete with humans late game in the use of the combat spells available...