2-3 tanks. Assassin wise, I mainly play kat, talon, and yasuo if he counts. Although yasuo does fine against tankier teams, he's just not likey to 1v3.
How am i supposed to teamfight with them? Assassins want to 100 - 0 people ...
Fondling Gems1 votesIn general try not to pick assassins into tanks, however if you do get counterpicked then you have a couple options to MINIMIZE your disadvantage, remember you are still at a disadvantage though.
If your team can face them 4v5 then sit around the edges and zone. Even if you aren't doing a ton of damage someone like Kat WILL still threaten heavily. Think about how much damage Kat can do to a tank, lets say 20% of their health very quickly assuming they are interrupting her properly. Simply by being nearby and trying to zone their carries you put a health requirement on all the enemies. Suddenly tanks have to back off earlier because they can't risk giving you resets. If you go in and get blown up though that pressure vanishes.
If their team has a much better teamfight than you guys try to splitpush. Someone like Talon can force 2 people to come to peel him off, a single tank/squishy can't handle him and while 2 tanks can push him off they can rarely kill him. If they peel off both a tank and a carry to try and kill you then your team has a major advantage.
Buy lots of control wards and clear out vision. Do everything you can to wipe vision off the map and prey on the enemy support when they try to ward it back up. A duskblade is extremely useful here as it lets you walk through wards without being spotted.
Overall you become very team reliant because you can't deal with tanks. You need your team to burn them down/peel them off so you can go to town and wipe the backline.
One other thing to keep in mind is that if their team is sudo-tanky due to a lot of actives like locket you can try to blow up the squishiest target a while before the fight starts to get them to blow their items. Only do this if you can escape easily with someone like Talon/Zed. Ultimately a support's cooldowns can be more important in a teamfight than a Talon ult would be, and if you find yourself able to do it consistenly spec into some CDR like 
which will let you do it more frequently, help you survive the short dive, and let you heal up on jg camps before the actual teamfight starts.
This Is Your Dad1 votesTry to capitalize on mistakes. The moment the enemy adc is out by himself splitting, BAM. You see a tower getting pushed? Use your ad to nab it and your mobility to get away. Kata is one of those where you might be able to use the e reset to dodge some cc or just combo so quick the enemy adc dies before anyone can react. As talon it's even easier with your wall hop and dash forward. In a teamfight if the tanks engage your adc go after their exposed adc because your job isn't to protect but to kill.
