Well, you could do a lot better with archer decoys, more mages. Only 3 for evocations are a bit to low. You need some more.
AI archer mostly only target the closest.
You could always combine different tactics. Have some heavy (with loads of HP) units for example trolls, and one earthquake caster. So you have a lot of already damaged troops, a lot off dead archers, and troops who can handle the enemy. (Cut of the retreat routes, so if you destroy the army it is totally killed (Ghost Riders can kill most normal unsupported PD)). Also you could soften up the enemy army with some remote attacks spells, while costly the
http://dom3.servegame.com/wiki/Flames_from_the_Sky spell followed up with a unit who can cast earthquake or something else battlefield wide would kill anybody not killed the flames from the sky spell. (Teleport them in to catch the army before it moves).
Also, look at arrow fend. Stops 80% of all arrows. Throw one of the earth army of spells to give prot 20+ and you can stop most arrows. (Against an air nation this is stupid of course). Also, getting a caster to cast the army wide mistform spell (forgot the name). Will prevent earthquake damage.
http://dom3.servegame.com/wiki/Arrow_Fend
The basic idea of killing chaff armies is to get more buffs and mage support. A few heavy troops, blessed with a proper bless, under relief, mass regeneration, mistform, mr+ spells, army of ... spells can stop any kind of unsupported army. They are force multipliers. Read this:
http://www.erfworld.com/book-1-archive/?px=%2F125.jpg Higher nr of troops additive, more battlefield wide spells are multiplicative.
and even SC's can be stopped with enough magical support. Spamming paralyze, petrify (works on any SC, the second paralize effect is not resistable by most units), blindness, some of the hold in place spells (see:
http://dom3.servegame.com/wiki/Key_s...ization_Spells for the low level ones). And getting some troops buffed with weapons of sharpness (weapons become AP, so only half the prot counts), to pound the immobilized SC will solve most SC related problems.
Of course there are other ways of killing large mage supported armies, esp if they attack (then you can get off an earthquake or a rain of stone or any other spell before the other side can cast defensive spells). But that is mostly remote attack spells, or tailored armies/sc to kill specific armies.