Eh... there isn't, really. Just about everything in the game like that has pretty clear warnings (zone entry feelings, vault warnings, popups, etc.), though it's entirely willing to let you go ahead and bumrush a gaggle of ancient wyrms if you're going to insist on it. Even RNG based stuff (rares, randbosses) have unique coloring and information on their talents and a bit on their kit just an inspect away. Knowing ahead of time certainly makes it easier, but in general if you're in over your head you put yourself there pretty intentionally, heh. Not too much in the game that isn't pretty explicit one way or another it's about to try abnormally hard to kill you. People just seem to like ignoring all the bits and bobs implemented to tell them to be careful, for some odd reason

Though yeah, if you were fighting the minion undead in the main chamber you were basically committing suicide. Dealing with them first is workable (usually a decent idea, even) if you can break line of fire with the boss by ducking into one or the side rooms, but trying to do it while in plain sight of the necromancer is just asking for a bucketload of arcane death to be dropped on you, heh. If you're stuck fighting in there you pretty much have to blitz the boss down if you're going to survive, at least if you're not silly overpowered for one reason or another.