Enemy death-complete level

In level 7 of my .wad, how would I go about the event of killing a -specific- Baron of Hell and ending the level (if it is possible at all) for JDoom? (if that is even possible)

Triggering an action when a specific instance of a monster is killed is currently not possible in jDoom.

Atm you can:
trigger an XG line when all of type X are dead.
trigger a console command every time one of them dies.

In 1.9.0 you'll have these further options:
trigger an XG line when there are no monsters of type X in the referenced sectors.
trigger an XG line when there are at least one monster of type X in the referenced sectors.
Compare a value against the number of monster type X and depdending on the logical result of the operator used in the equation, trigger a console command or XG line

As jDoom uses the Doom WAD format it isn't possible to give individual TIDs like you can in a Hexen map.

Hmmmm, alright maybe I'll try the XG line, thanks DaniJ

Since you are using map07, you could try one of the other special tags. This map I was trying to make work in JDoom as it did in other ports I tested - http://numerometria.com/7TEST.ZIP This map still needs to be fixed.

Graf_Zahl showed me how: The backs of all the lower sides of the special tagged sector have to have a texture (even if they are never visible) or JDoom won't respond the way I wanted.

Here I have a hellknight that rises out of a pit guarding the exit switch. If you make it a tighter area, the player has to get through him to get to the exit.

Upon reflecion, I find that I have no idea what an XG line is or how to use it... help?

XG is a way for modders to create new line/sector specials for use in their maps.

Have a look at the WIKI and the original design Reference. If you need any more help just ask.