Sounds like an impending deathtrap, Eternal. Kind of that "better not be around that when it erupts" kind of thing.
I agree with EJ; there ought to be more difficult bosses not because they are bulked up and have an insane health/power stat but rather their interesting attributes. Just as Riss said about the Pentagon Thief, he is a epitome of unique boss creation. He doesn't attack you; however, he steals your weapons, and speeds up as he gets hurt. And the payout for taking him down is your guns back plus a Max Ammo (or Bonfire Sale if he dies without snatching anything).
In that case, there are particular variables you could consider in an enemy:
Speed
Fast and difficult to lock onto, but has mediocre health levels and doesn't deliver much damage to players. Better to take down with a group.
Slow, but a complete brute. High health stats and damage outputs enough to make you go red in one hit. Of course, attacks are sluggish but requires a great deal of teammwork to take down.
Defense
High defense stats, but is fairly easy to wipe out with joined teammwork. Loading a great deal of gunfire for that finale explosion/defeat.
Low defense points; however, the player must shoot a certain point to actually deal damage. Given that, it probably will not be easy to lock onto.
Changing defense mechanisms will create an "armored" boss but upon certain circumstances (i.e. getting hit by explosive in certain way) will release its weak point, allowing players to nail into it before locking back into the armored position.
Attack
Volatile attacks can be generated in several circumstances. 1) When a player is too close to it. 2) When a player is stationary for too long. In these situations, looking for the critical point is a good idea. These kinds of bosses won't have a large attack radius but their hits will be fairly sluggish so timing is about everything.
Weak attacks, but quite consistent. Suppose that these bosses generally move quickly, and will attack you whenever possible. Shouldn't bring you down instantly, but taking more than two hits at a time will be your demise with how fast they attack.
Enemies that don't have any attacks would be unique. Like the Pentagon Thief or an offset of the Ghost in Buried, they will tend to have a unique ability such as stealing someone, switching up players' locations, altering their points, or making a player zombie-bait (if the boss is mid-round).