So you're going to give Exp and EVs? You're so generous! If you have Latias and Latios on Emerald, though, and can clone them, they learn their sacrificial moves relatively early on, Latios at 5 in Emerald and Latias as a base in XY, and that would everything needed for the EVs. So really, if you want to train them, you could just train them on Mt. Battle, clone them, and send them on up.
Taunt may help the Wishers struggle themselves to death, but Metagross would be the wrong way to go. A 0 Atk- IV Golem would be better in that respect, as Selfdestruct can be bred onto Geodude, and the Golem could be achieved a minimum level of 25, as opposed to a 0 Atk- IV level 45 Metagross, who learns Explosion through a TM. Neither set is ideal, but the Golem is more survivable.
No, the best answer, I think, would be Belly Drum Poliwrath with an even Hp stat, where the trainee uses something to reduce his attack the same turn. It's not ideal, but if ...
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So. What do you think the mechanics of Exp. Share will be here? Ugh.
If it's an Exp. All again, ignore the Poliwrath, go with Metagross or Golem, your pick. Use a ghost.