While it's recommended to do this PvP and getting bounties / attacking 1K Players.
You can do this during campaign Lvl 1-1. CPU uses 'dagger units' which require 'lightning' or 'sun' (social unit) units to beat.
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 200 energy and gains 500xp & £10,000.