[thejeff]

I usually use impossible AIs. These days I've been playing smaller maps so there's more action early on when the AIs are more effective.
If I'm rushed by huge AI armies from multiple nations early on, I'll probably lose. If I get time to grow to midgame, the AI stops being able to compete.

As far as MP goes, I've never really been a factor in the late-game, so I'm mostly judging from commentary here. Bear in mind you won't be the only one wishing in MP. Arcane Nexus is one of the game-ending spells. Don't cast it in MP unless you're actually intent on declaring war on the rest of the world. Clamming and eventually wishes for gems are the usual way of funding wishes.
Magic power is generally not considered a particularly useful wish, sure it would take far more gems to duplicate without wish, but it's not all that useful to have 1 caster with 10s in every path. Hardly needed for rituals and if you send him out on the field, even fully equipped he'll likely die in a battle or two. Sure arithmetic shows it's powerful, but is it useful?

Everyone will have an end-game strategy. Using clams to fund wishes, mostly for Seraph class SCs is one of them. Maybe even the most effective. Clams require a lot of investment upfront and can be hard to mass if you're forced to fight most of the game.
Blood nations have their own paths. Tartarians are still effective and cheaper.