You need to make the farm freeze just in front off the tower so he cant farm and you can you need to do that at the start until you go back wen you go back you need to push so the wave is going his way at the turret and if you got your first power spike you need to play aggresive because if you did the freeze right he has like no farm and you much and to freeze dont auto attack at the start and just let him push dont help your wave and wen your wave died and his wave is going to crash in your tower go stant in the group minions so they dont go n turret and wen you got them there only last hit.

just poke so they cant farm because your Q has much more range than the most marksman so if you do it right and make them low. Wen thay are low enough than E over minion at pick up the kill.

Wen you play vs Hard tank like sion or ilaoi you need to keep hitting hem with mini gnar because of his W you do more damage and the runes you can beat them more easy but keep poking and use the rune good and try to freeze the lane so it is harder for them to farm

https://www.youtube.com/watch?v=-SpPe_kfLVE
https://www.youtube.com/watch?v=QlBSroJN4Ic