Coop vs AI is very good for leveling. It gives you basically the same exp as a normal win.
[From level 1-9, play beginner coop because it gives same exp as intermediate but it's easier and faster.]
[From 10+, play only intermediate because beginner coop gives you 15% less exp now.]
Level 1-19: No limit on coop/custom games so ONLY play coop. Make sure you pick a ranged AD so you can carry in case your team is full of beginners (I have lost from people that afk but don't leave, and having 2 lost lanes. If you pick an AP carry, you may not be able to 1v5 as effectively as a ranged AD can).
Level 20-30: Play your daily coop games until you run out of minutes (each day you get max 180 minutes for coop/custom, when you run out, you get no ip/exp reward). This should be 6 or 7 games. When you no longer get exp from coop, just play normal games for the rest of the day. I'm not sure if dominion gives you faster exp, you can experiment and find out by yourself.
This should only take a few weeks if you use exp boost.
