If you can make a lot of tokens (say, Mites), Flesh-Eater Imp can become as big as an Eldrazi- an Eldrazi that can instantly end the game for someone
Mana produced by Overgrown Zealot's last ability can be used to pay morph or disguise costs when turning a face-down permanent face up or to pay costs associated with turning a manifested or cloaked permanent face up
Mogis, God of Slaughter and Sygg, River Cutthroat are the first that come to mind
Here's the list of trainers I work with to accomplish this
Building in other ways to create artifact tokens can give you a good supply of things to sacrifice, and blue has a few ways of untapping creatures that you can use to get multiple activations off Bjorna, Nightfall Alchemist
PvP with random players is also effective for leveling up, as there's no limit on daily games, and each victory earns you 15 EXP