Simple premade and obviously copy/pasted idea from other games.
Juggernaut:
An NPC will be the Juggernaut though (Omg Michael you cheap fucker I wanted to be the Juggernaut, what? scared of my uber skillz?
Reply: No I just want a PvE Boss fight for once where everyone can contribute to bringing it down, making a player be juggernaut would only make it impossible to beat)
The Juggernaut will be a SuperMutant who likes to switch between a rocket launcher and a minigun, has 300% in both skills AND FA, has 2000 Hp, normal Damage resistance e.g. 40% like every mutant, 10 in every S.P.E.C.I.A.L and unlootable so it can't run out of ammo.
Location can be anywhere GMs feel like.
The reward for the PLAYER(S) [notice the S, this means for everyone who contributed and survived killing the Juggernaut] will be rewarded with a Minigun, 300 5mms ap, BA, 10 stimpacks, and two halfway levelled up Mercenaries of GMs choice [Due to having kiled the most powerful npc in the game your bound to get some followers in RL who worship you anyways if it was RL stuff]
Notice: I only made this because the server was turned off so if it turns out to be shit, my apologies.