Hey everybody!
This is an easily obtainable achievement that isn't connected with any story missions. So, what do you need to do?
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Some preparations : go to your safe zone, open with
your quest board ( in the middle, hard not to see ) and pick some easy quests that have stars as a reward. Usually it's 3 stars or 10 if it's an epic quest but that one takes much, much longer to complete. So take the simple ones. Doesn't matter what side you'll be playing and completing them, in your Backyard, private game or ranked multiplayer. You need to have 5 stars for this quest.
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Once that is done, get out, go to the flag, if you're coming from zombie side, take a left at a flag, if you're coming from plants side, take a right. There is a hidden entrance to the slums. Enter and go through the little maze. Go down one level. You'll see a fish lying on the table with a big exclamation point above it. It's right next to the Crazy Target Range entrance, by the way.
Approach the fish, press
and it will speak to you, offering to train you ( basically give you XP ) for 5 stars. Approve the offer, get your 750 XP, the achievement will pop straight away!
Hope this helps! Have fun!