Assuming you were able to shut Teemo down in the laning phase, you can now burst Teemo down quite easily 1-on-1. Just remember to get your team to place pink wards around baron or any locations where you are looking to catch an opponent out to keep his shroom count low! Congratulations, just make sure he doesn’t free farm and clear shrooms, and Teemo is now a useless member of the opposing team.
If you weren’t able to completely shut down Teemo, chances are he either is fed or has a fair amount of CS . If this is the case, it’s really important that as you are traveling in groups trying to catch people out that you have a few team members with pink wards. If you have one person, the only shroom clearing ability you have is wherever that one member puts their ward, and team fights will be difficult with the extra damage and slows if you end up fighting on shrooms. If you have more pink wards, you can keep the shroom count down as the game progresses, and you can even use the wards to catch people out, as it takes a while to destroy a pink ward. Teemo probably isn’t doing very much damage apart from shrooms in this mid-game, so it’s important to make sure he doesn’t have many mushrooms on the map.
If Teemo is doing a lot of damage, you should treat him as an opposing adc and focus him earlier rather than later in a team fight. Try to make sure the person focusing him doesn’t rely on autoattacks too, since Teemo has a blind. Teemo will not be very tanky and can be quickly focused down at this point in the game. As always, look at builds to figure out who will do the most damage, but a fed teemo will not be going tanky – he will be getting damage.
Another thing Teemo may be doing during mid-game is split pushing because his Move Quick allows him to escape easily. All you need to do is send someone/an assassin with burst damage to the lane he is pushing in order to scare him away or kill him. Always make sure this person walks through clear areas or brings a pink ward with him to avoid chasing Teemo through shroom fields.
Regardless of how you may or may not have shut down Teemo earlier on, he will be pretty strong and his mushrooms will be really annoying at this point in the game. Continue to have at least multiple pink wards around the map to stay cognizant of mushroom fields, as a fight in a shroom field at this point in the game can mean very easy Ace against your team and a push-to-win scenario for your opponents.
For team fights, you still want to focus Teemo, prioritizing him, the adc, and the apc as primary targets, since they will all be dealing tons of damage at this point in the game. Teemo will be particularly easy to burst down if you can get some hard cc on him, since he doesn’t have much to get away from hard cc. It’s easy to disregard Teemo, but you need to remember that in the late-game, he does similar damage to the opposing adc and should not be ignored.
If Teemo is split pushing, your entire team should go after him, since it might be common for the opposing team to use Teemo as bait and set up a trap for your team if they only send one person to deal with Teemo. Make sure the map is warded though, so you do not send everyone for Teemo while the opposition sneaks a baron. Map awareness, as usual, is important for dealing with Split Pushers, but if you can kill a split pushing Teemo, you can win a fight versus the other team, since the only front line they will have is possibly a jungler or support (as Teemo went top).
Teemo is incredibly dangerous and annoying in the hands of the right player. Since you don’t know what kind of player you will be facing, it will be best to counterpick him by choosing a champion with a gap closer and burst abilities.
During the laning phase, it is important to manage your trades with Teemo. You don’t want to get forced out of lane by being too aggressive without damaging Teemo, but you don’t want to play too passive to let him farm a lot either. It’s best to have several all-in burst harass encounters to force the Teemo out of lane instead. Use a pink ward in the river once he hits level 6 to prevent mushroom control over ganks and also get a little extra gold from destroying shrooms. You can also bait him out if he tries to kill the pink ward.
During mid-game, send a bursty assassin to deal with Teemo if he split pushes, make sure you have the jungle scoped out with pink wards. Focus Teemo in team fights by locking him down with cc and bursting him quickly.
During late-game, send the entire team to deal with Teemo for split pushes and keep baron warded. Make sure you have a few pink wards around fight locations and focus him in team fights as though he were another adc. Never fight in shroom fields or if there is a risk of shroom fields. Use pink wards to scope out potential fighting locations before engaging there.