Worms don't surf. Sounds so simple, doesn't it? Let two stupid hunters leap off the ledge. Well you couldn't be wronger.
Thanks to 343 tinkering with the AI, the formerly most powerful infantry in the Halo universe now RUN AWAY from you when you're attacking them. Maybe they're afraid you'll do what you did in halo 4? Maybe they didn't really like exploring the floor, maybe they simply just don't like the water.
Anyway, you're stuck. The AI retreat and you really want this achievement. Maybe you have a buddy with you but you're grinding away only ever able to get one down.
Well, I have a solution for you. Tough Luck Skull.
I'm not kidding here folks. TL makes the enemy refuse to retreat in combat, your Hunters will have no cooldown on their melee swipes and will charge at you like the good ol' days of yonder past.
Put it on, come near a hunter and lure him to the edge, either with your buddy or just use your hover near the edge of a ramp so the hunters will lunge off the edge while you're jetting. You'll find the hunters happily suiciding within the first move or two.
Shouldn't take you more than 10-15 minutes using any of the previous methods as long as tough luck is on. Just remember to make sure to save your last checkpoint before you begin in case your AI decide to ignore instructions and attack the Hunters too. Good luck and Happy hunting.