I've never played Tien Chi, but...
Legions of Steel can boost your regular troops' armor a bit (+3 per armor piece, reportedly, but it targets commanders with an AOE, rather than targeting troops, so you may have issues getting it to actually hit your troops but can potentially solve them either by embedding your mages or embedding dummy commanders for your mages to target). (I don't know if the other aoe spells work the same way, but should probably find out, heh.)
If you go for alteration you eventually hit Iron Warriors (Alteration 5), which sounds much better than Legions of Steel but gives your troops an elemental weakness that your enemies could potentially take advantage of (extra damage from lightning). You can, however, combine it with legions of steel. You also get Marble Warriors at Alt 7, which is stronger, and has a different elemental weakness (extra damage from cold).
Curse of Stones was unimpressive when I tried it (too slow to be useful, considering how fast the killing happens). Destruction's potentially useful, but it won't work on anything with magical armor, which means it's useless against SCs and the various summoned troops etc which come with magic gear. Same for the one-person variant of the spell with the confusing name... Armor of Achilles? Yes. You'd think that would make you invulnerable except for your heel, but nooooo.