Gnar is a very combo dependent champion, even more so when his passive Rage Gene has activated and his has transformed into Mega Gnar. When in Mini Gnar you want to poke with your Boomerang (Q) and then land another two basic attacks for your Hyper (W) to activate dealing bonus Magic Damage) When they are low enough try build up your rage ad fast as possible, when turning you will notice your Hop (E) will be glowing blue, use it to engage and you will still bounce on top of an enemy and instantly turn into Mega Gnar. From there use Wallop (W) then ignite them and hit them with your Boulder Toss (Q). By then they should be dead and if not do whatever else needed.

If you are feeding the other team there isn't much else you can do other than farm safely until you have caught up