How to Utilize Gnar's Switches to Mega Gnar

I'm really bad with Gnar right now, he's just not as intuitive as some other Champions I own. In particular, I often end up turning into Mega Gnar when the enemy has more health than me (Even if I get the health boost as Mega Gnar) and I can't engage, especially if there's a really tanky top laner going against me.

I know I can jump out and smash champions against walls as Mega Gnar but usually it still ends up with me lumbering away at a snail's pace, tail tucked between my legs, because I'm trying to engage tank v. tank. My highest grades with Gnar is around a C or C+, and I'm not improving much.

I tried watching the SKT versus SSG game to see how Gnar was played, but it seemed like even then, Mega Gnar was more a disadvantage than an advantage against Kennen.

Any Summoners I should follow who main Gnar especially well? Or should I consider trying Rumble or another character for top?