In the beginning of the game. Try to not get in a fight with your enemy. Stay back and farm for a bit. You poke with Q a little bit but if you harass them to much they will start to get back at you. Once you have part of your defensive item now you can finally start bulling them.

By this time in game, you'll be transforming into Mega Gnar quite often. While in that form, you deal a lot of damage to your enemy and take a lot of hits too. If your enemy is dead or back at base, feel free to help an ally lane.

Finally Gnar is at his full potential. You'll have low cool down reduction, have lots of mobility,(even Mega Gnar has some mobility) deal high amounts of damage, and be able to take hits. When team fights occur make sure you engage when your in Mega Form to take the hits for your team and deal lots of AoE damage.