DS Jumpluff would be great but it doesn't get Light Screen :\ (according to
http://www.smogon.com/dp/pokemon/jumpluff/moves)
Try and find out from your initial switch which set they run. You can't. Every time Venusaur switches in you have a very, very large predicament. Think of what you just said. I'd switch in Altaria first, than Arcanine, or maybe SubRoost Moltres. Common thread: each one of those are weak to Stealth Rock. You can't just go blindly switching to random specific-Venusaur-set counters because they are getting hit, usually super effectively, by Stealth Rock every time they switch in. Something is getting incapacitated for a while too with Sleep Powder somewhere in the middle there.