Dungeon Enemies' Spawning

A short poll in relation to this map. Just how should I handle enemy spawn camps in a 4-player co-op dungeon?

• Pre-placed units:
  • Pros:
    • I can easily control item drop-rates for pre-placed creatures.
    • I don't have to assign any variables to target pre-placed creatures.(unless I, you know, replace one or something)
    • I know exactly where they are on the map.
  • Cons:
    • Two non-allied factions may engage each-other before I want them too.
    • Sometimes, ranged creatures will fire through walls to aid their partners.
• Generator Spawning
  • Pros:
    • Hordes of enemies can continue to spawn while one or more control units still live.
  • Cons:
    • Hordes of enemies may continue to spawn while one or more control units still live.
    • Giving spawned units any Bounty Reward (or allowing them to drop items) may encourage Spawn Camping.
    • Special cleanup triggers may have to be employed for spawned units and their corpses.
• Unit Enters Region
  • Pros:
    • I can specify what units spawn, with triggered SFX to boot.
    • I can give spawned units an order to attack the Entering Unit.
    • I can close and lock the doors behind the player.
  • Cons:
    • I have no clue how to make spawned creatures drop random items.
    • Special conditions have to be placed so that the player doesn't abuse the spawn system and farm the creatures.
    • I have no clue how to handle this type of spawning in a non-linear dungeon.
• Other: Lol, wut? Please specify.

All of the above. A healthy mix of how enemies spawn will make the map feel more dynamic and will get the players involved at different levels in different parts of the game, making it more replayable.

Agreed, infact Warchasers which is a pretty basic dungeon uses al least 3 of the above methods.
Im pretty sure all of us can do better than that.

Ok, so such units as pirates should be pre-placed? Undead-type things I could give a one-time spawn trigger, then keep them in the room they spawned with a periodic trigger (and I'll be cheap by giving certain level undeadish things a modified Reincarnate). I guess I could make use of a cut-scene to force all party members into rooms containing a mini-boss/boss (to shut the door behind them ). I'll save flying creatures for floors with outdoor rooms (F2 and F3 for me).

Right, I've got torturous plans in me head for the adventurer(s) that will be traveling through my dungeon. Thanks a bunch guys!

I would combine all three and use them whenever it's useful

OR

having some preplaced units in an area, when a player unit enters, spawn some additional creeps (reinforcements) and turn on a creep-generator
would be really cool e.g. in an evil castle or town

Well just sue the system you feel most logical in the situation.

Prespawned units will be good for a town or base.

Spawning units will be good for a graveyard with skelez burtsting out everywhere.

Go with a Left4Dead spawn system. In the background track how far along the players are and how well they are doing, HP, Kills, Level etc, then spawn the enemies ahead of the players or where they cannot see.