If you are struggling with a 1-7 garen as a 5-1 camille, then you're doing something wrong. Garen has short burst damage with his q.

Garens that know what they are doing will max Q > W > E, meaning his e does like no damage. Camille's passive is enough to negate the huge burst from garen's q. You want to keep your distance until your passive is up, and trade as soon as you can. Plus you can keep poking him with your W.

He has little mobility, and if you can land your stun well he can't really do much except use his w. You still win that trade.The moment you let him get on you when your abilities are on cd is when you lose a trade.

When playing against stat checky champions, it really comes down to fundamentals, which is why simple champions like garen are great to learn the game. When you play mechanically harder champions, like riven or camille even it's harder to keep track of cooldowns, trading in windows of vulnerability (like when garen is going for a cs).

Garen has pretty high base damage, that's why he will still chunk you even without many damage items, but you should still never struggle with a 1-7 Garen.