The Hard Luchadore Mayhem Activity:
If you have already completed the mission "Air Steelport", then that island is overrun. This makes it a bit more difficult.
So you need to get 250,000 damage within timelimit.
But I can give you some tips on completing that one.
At the start take your rocketlauncher and shoot the gasstation (the gaspumps).
after that shoot the fences across the street and make your way to the planecrash site. On your way you'll see a high brick wall with fences on it. Shoot those.
Then go to the road adjacent, there will be a lot of fences in the street, and some propane tanks.
First walk up the street and then walk down the street while shooting all fences. Normally you should (almost) have 250,000 damage.
at the end of the street turn right and just keep shooting the fences.
I got this one with even over a minute left on the counter
Another way to get this achievement easily is by using a VTOL during the activity. Thanks Lofwyr2030 for this hint.
I hope this is helpfull!
And good luck!