Props to D3ST1NY and xTCx; both good guides. Unfortunately for me, I couldn't get it very easily as timing the jumps is a nightmare.
Even if you're doing it alone, get a second controller. Walk them both to the terminal and leave the arbiter a safe distance from the jump. He will act as a checkpoint to spawn back to. This took me about 15 reloads to realize.
Once you are both there, try xTCx's rocket jump. I couldn't do it but you may nail it. If you don't, you'll spawn back without the rockets but 2 grenades of each type. From there you can try the double plasma technique from D3ST1NY; which worked for me.
Once you're are on the ledge, kill the arbiter and stand him on the ledge behind you; again he is a checkpoint because you'll probably fail this jump. Keep trying until you get to the ledge. If you're using a second account that needs the achievement, just kill the arbiter a second time and he will spawn on you as before.
Grenade Jump Tips--
-Jump Earlier than you may think
-Use the Cowbell Skull
-Run sideways, jump sideways, spin towards landing pad
-If you fail, change one variable at a time so you can track what is going right and going wrong; ie height, distance, velocity etc.
Shout out to Maka for the inspiration to use another account. If all else fails you may be able to use his method with a second controller. It would take some clever hands though.