It's a matter of team comp. Some tanks are better at engaging, others better at peeling, and protecting. If we looked at support tanks, it would be the difference between a Leona, and a Braum. Another example is that Maokai can target his Twisted advance targeting an important target, while Sion's Q and Ult affect the first person hit and who ever's in range.
Also depends on whether you have a good pick mage/assassin. Because Zed, Leblanc, Ahri will do a better job killing the ADC than the Tank.
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NO, you can not, because their tanks will just go for you and focus you down
Because the ADC is likely the only person on the team who can reasonably get LW and BotRK means that they are the only ones who can kill the tank. But the trick is to focus it, and when. If you engage, you don't want to target the tank. But if you get engaged on, (an the rest of the enemy team isn't less than a few paces away), then the enemy tank will die to the focus fire, before the rest of his team can initiate.
Summeries my points, If you can jump the enemy carries, fine but make sure you can actually kill them. Otherwise your best off clubbing the closes guy you can reach.
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You HAVE to focus the tank if they are in your face. You cannot ignore them when they are so close to you. People will search for reasons to flame you though, which is often "OMG YOUR FOCUS SUCKS" because it's really easy to blame.
welcome to YoloQ, Where no one adapts to changing conditions, and blames it on the Jungler/support for things not going their way. I had one game where the entire team was complaining about the enemy being too tanky. So me playing the ADC adapted, getting BotRK, and LW. But the rest of the team could not adapt (i.e. they didn't protect the tank shreder.) They rather dive the back line, then wonder why I couldn't escape flanks, or kite the enemy tank who did the flanking.