If Teemo is on the enemy team, here's a tip
STAND STILL after a fight.
If you didn't bring your sweeper, or it's on cooldown, do not risk walking around and stepping on shrooms at low HP. You're gonna have a bad time.
Unless it's absolutely necessary that you move, try not to. And specifically avoid bushes.
Edit: This is for when you can't afford to buy the MR (either because Teemo is extremely fed or the entire enemy team besides him is AD. And ONLY when you're low enough that a few shrooms can kill you. It doesn't matter if you killed him and still have 80% HP.
And for beginners, obviously. Newer players are the most likely to run into Teemo due to his price. I'm obviously not pandering advice to long-time players.
