after you get warmongs armor and maybe thornmail you should be able to kill solo rammus and you will still have 1/5 HP

early game try to last hit minions with basic atack. use majestic roar if enemy minions attack your tower or if it rarely happening you can use it to poke enemy chap.

in early teamfigts you go infrint of your team and when you see enemy ad or ap carry wondering too close to you, toss him with rolling thuder. after that use all of you ability accept frenzy. save frenzy for trying last hitting opponent. you should be able to go 1vs3 if everything is going good

You should be able to go 1v3 if everything is went good even against rammus, pantheon and master yi all together from start.