Worms Ultimate Mayhem

Easy to boost. Get a friend to group his worms together, and fire a poison arrow on the floor to poison all his worms. When their health is low (below 20), simply airstrike the group.

Ok so this IS a lot harder than i thought at first. Probably easier to boost rather than wait for the "perfect storm" situation where you can pull it off. You need to kill 4 worms minimum in one turn and your own worms do not count. I put myself in a pile next to 3 worms and drowned us all and didn't get it. I recommend setting up your team weapon as an airstrike with all 10 drops that push worms a lot so you can try to drown a pile when you find one... all 10 drops gives you a wider spread to do damage and/or push. Other tricks for getting multiple kills:
1) Poison, poison, poison! - the more worms with 1 point of health on the map, the easier it is to get multiple kills
2) Ropes can be used to move barrels and/or mines onto enemy worms and get kills without even technically using a weapon yet... so try to kill 1 or 2 this way and then use an airstrike elsewhere on the map if you can maximize your time
3) Grave damage from a dying worm can cause over 25 points damage to a closeby worm so, again in a perfect storm situation, you could shotgun 1 worm onto another, low-health worm and then hopefully do the same elsewhere... this requires at least 4 worms with 25 health or less in close proximity

i'm not gonna lie... all this talk might help you... but in the end i played 30 more games after winning my 40 ranked just trying to get this achievement and finally gave up and boosted it