Now this achievement seems annoying right, the easiest way is obviously find a friend and trade favours, you help them get it by you using any weapon and then they help you. But that's annoying. So here's a better solution.
Find a friend who hasn't done Leave It Where It Lay.
Now what you do, is you do that trade thing, but
you two do both achievements at the same time, one each.So what you do is you find that friend, you can't kill grunts, and they can't change weapon.
For you:
- You can use skulls, even Bandana (This is really helpful).
- You can use any weapon you want. I recommend sniper and shotgun, try to avoid the rocket but it is helpful too.
- Be careful with killing things accidentally. Explosives are dangerous, as is vehicle physics and the sniper can shoot through enemies.
- You can kill everything
except Grunts, this means sentinels and banshees. However, be careful, these could fall and crush grunts. I do not know if this will count as a splatter, but better safe than sorry.
Advise your friend:
- They can die, yes, even if they respawn and have an assault rifle instead of a shotgun, just make sure they don't pick up the shotgun they lost.
- They can use vehicle weapons such as the ghost or warthog gun.
- They can pick up grenades and use them just fine.
So yeah, the general strategy is pretty easy. Both of you can work together rather than one taking a backseat. One just has to avoid killing grunts, and the other can kill them, the other has to avoid picking up weapons, and you are free to do so. If both of you don't have the other achievement, then you can trade and do the other achievement next where you two swap roles but still get an achievement.