Travel to Tundra Express

Jump off the first cliff to the right and into the two varkid spawn areas

Wait there with the varkid attacking you (do NOT die, unless playing multiplayer)

Varkids will evolve into Adult varkids, then badass varkids, then super badass, then ultimate badass, at which point it will attack anything, like a goliath, after a bit it will transform into vermivorous.