Seviper in its own deck isn't good. The 90 damage is nice, but isn't worth it. Seviper's recovery isn't great, since once one Seviper gets KO'ed, you'd have to setup another Seviper and attach two Energy on Seviper to do some decent damage. With only 90 HP, Seviper is gonna go down pretty quickly whether or not Belt is attached. Also, if the Skuntank G is Sprayed or if your opponent controls the Stadium, then you aren't going to be doing any damage at all. Seviper would have to take 20 damage just to do damage during your next turn.