The other solution is great
but it also misses a few key points to obtaining this achievement easily.
Aiming to obtain all achievements with minimal effort i suggest building alter of the gods. Shock is a great tool that can allow you to beat normal enemies every time. Immediately use shock, then summon your ally, and if a normal enemy you will probably use 1-2 more shocks to win without taking much damage. Shock II available at level 21 will stun your enemy for 6 seconds, then 5 and ect. This will make it even easier, although more expensive, to claim bounties. This is especially effective in gaining air unit kills while aiming for bounties/player attacks.
I have 300 bounties and I don't play more than 5 mins a day. I like games with daily rewards, i farm them every morning. I do leave it open while i play certain games(Lego games have long load screens/cut scenes which make it optimal to farm games like these) so i can collect gold, i have 3 acropolis that make this super fast, and it helped level me up too.