Sniper Caitlyn is a build that I always go for mid. At level 18 and full build you will have your ultimate evry 22-23 seconds without dragons. Also lethality build on Caitlyn can melt every weak mage on mid lane, so its a good option if you pick second than your opponent. Try recalling every time you get kills so you can come back more powerfull.
Your ultimate is your stronger attack. If you have opponents like yasuo and samira use your ultimate wisely at the correct timing.