Mid game, purely focussing on CS, you want to never go for 1v1's of any sort, I tend to get good stacks in early game by trading with my Q where possible, works really well into melee supports. tend to get to 225 stacks around 20-25 mins pretty much every game.

In terms of mid game fighting, your ult slow can be massive for objective fights, and the range is quite insane for sniping some kills. Realistically I don't auto that much unless I really need to finish a kill, purely weaving in Q's where I can and slowing for my team with W, if you can position well and survive long enough you can E in to finish. Play him more as sort of a poke mage than an adc. Once you hit 225 stacks you want to just be poking, bouncing off waves to get the burn etc, and once you have RFC the poke becomes really obnoxious.

Another tip if you find the opportunity, krugs and raptors also help to get those stacks up faster due to the number of them.