While it's recommended to do this PvP and getting bounties / attacking 1K Players.
You can do this during campaign Lvl 2-4; Lvl 2-3 is 1st to have air units, but CPU uses 'dagger units' which require 'lightning' (last air unit type to unlock) or 'sun' (social unit) units to beat. Lvl 2-4 CPU uses 'alpha' units, which can be beaten by 'dagger' or 'sun' units.
As with all campaign levels, CPU spawns up to 5 units, thus a minimum of 100 repeats are required, and 'auto-fight' also works!
Based on 100 plays = costs 600 energy and gains 700xp & £85,000.