I got to the slime escort level, and I am really not seeing the frustration.
First of all you have a big count down, tells you when the slime is coming. About 2 minutes before I run through and murder every hero in the dungeon. Then when the slime comes, I have plenty of time to escort it, no heroes escape. As for the level taking too long, sure that can be annoying, but every theme park game is like that. You are always waiting for something, whether it is another hero or just the value of your park/hospital/zoo to increase.
You really really don't get penalized THAT badly for killing heroes before they are totally happy. I don't even really notice it when minions kill heroes. Soul energy is pretty important in the first few minutes of the level but after that it is just farming for prestige. In the first few minutes it is also really easy to keep track of heroes. Nenjin's graph is right on. The first few minutes are chaos and may cause you to restart the level once or twice. After that, it is pretty smooth sailing and actually kind of fun.
Two pieces of advice I’d give though, are sprinkle gold all over your dungeon. Let the heroes wander where ever they want and don’t try and keep them on a set path. Put up minion spawns AWAY from any item that needs interaction, at least 4 or 5 squares away. Otherwise the hero will go start to gather gold or read books, sense a monster run over and kill (without it causing any damage to him) then go back and start to read or collect only to have another monster pop and the system repeat.
Also concentrate minion spawns so even if the heroes tear through them the minions get a couple of hits on the hero. I generally put the pentagrams around a corner so the ranged heroes can’t just farm from afar (even though they mostly just want to damage).
Never put any prestige items near your dungeon heart, only in the places you want the heroes to roam. One of the early levels has two shelves and a couple of lamps which always manages to distract the hero right to your dungeon heart, where they get board and attack your avatar. I am not sure what causes this (maybe more prestige per 4x4 square or something) but right now I just avoid it.