Ezreal and Twitch make up a great early game with Swiftshot, and you can build your frontline based on what you find. Level to 4 at stage 2-1/2-3.
There aren't too many notable 3-cost units that help out too much in the mid game. You can add in units like Bruisers, Cavalier, or more Guardians but the big spike will be when you hit a 4-cost unit like Xayah. Level to 6 at stage 3-2.