When playing gargas, you have to play pretty aggresive in lane and always poke with your Q, and if they try to poke back you just use your W and your 3 hp pots. But something to remember on Cass is that your passive gives you free gold so when you lasthit you get a lot of extra gold which good to get extra items, to make you stronger. As soon as you hit level 6 you get your ultimate which makes so you can trade better because you can bait them, so if you get low you just ult yourself and get 1.5 seconds of no death.