This achievement requires you to beat players with a bounty 500 times. To find players for a PvP battle, click the fight icon on the bottom right of your screen. Players with bounties will have a gold icon and a number next to them. As a general rule, only ever attack players marked as 'Very Easy' or 'Easy'. I found myself losing to normal players quite often, which isn't worth your time. Be sure to be vigilant by clicking everything that appears on screen, and swiping when the enemy uses boons. (Catapults, flaming arrows, etc.) Once you reach level 12, you can buy a Grounds of Aether to train air units. Do so immediately. I find that air battles are marked as easy far more often than land ones, especially when you fight players who haven't reached level 12 yet. I also recommend playing the free lottery every day, as you have a chance to win very strong units to make this a lot easier. You should also try to fuse as many units up to level IV as you can, preferably ones with a high defense like Sorceresses. Fusing can be done in training buildings by selecting the Fusion panel. A fusion needs 2 units of the same rank. (E.g. 2 Slingers III units.) If you can't find any enemies with a bounty or these players are too strong, try placing bounties of a 100 gold on other players. This will make them dissapear from the map, to allow for new players to appear who might be easier to beat.
I found that winning in PvP a lot would rank me up which gave me way bigger rewards from bounties. While I heard people say it's best to stay at a low level, I highly advice not to, as the bonus gold makes your life a lot easier.
One last piece of advice, join a big alliance. This lets you call alliance allies for help, which can be done once a day per alliance member and is free of charge. It makes battles a bit easier. Alliance wars are also a source of both easy and bounty-containing enemies.
I reckon this achievement takes about 10-30 hours of fighting should you find enough players with a bounty. Just keep checking often and you should have no real issue getting this.
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