Why does Gnar have a speed boost?

A percentile health shred, a ranged slow, and a jump with the ability to go twice the distance, as well as enormous defenses in mega form. Where is vulnerabilities. When is the time to engage, because as soon as you back off he gets to pressure you with a slow and health shred, and if you can't stick onto him well enough you get kited and shredded. Take too long and he gets mega with a lot of damage. When is behind he gets to farm and harass with a very long ranged skill on a fairly short cooldown. This only gets worse once he achieves black cleaver and gets stacking move speed amplifications, and a lot of cdr. The only time I ever see die is when he gets dived. If his mini form is supposed to be so vulnerable why is it that I feel the only time I can take him down is when he is in mega form.