Have it drive along an area on the outside of the map, and when it gets destroyed, a grate below it collapses or something.
Edit: I do technically know how to map (to some extent) and I’d love to create a map for this type of gamemode, but I don’t really have the time, unfortunately. That said, it would be great to have a template map with the I/O all set up, so that you only have to add in geometry and move the spawnpoints and such.
About the fading:
It seems that the NPCs (the Abrams in particular) have a “Fade Corpse” flag, which’s probably editable (duration-wise).
Ideally you’d make it so that, when destroyed, it only explodes into a whole bunch of gibs, which then eventually fade out. But I’m not sure that that’s even possible with only map stuff.
Maybe have the tank stay in one place, then when it gets blown up, instantly kill the tank remains and then activate a separate entity which makes a whole bunch of flaming metal gibs.
Is the tank corpse a ragdoll? Perhaps you could parent an explosion to it that throws the entire tank corpse out of the map, and then fade it out. It’s over the top, but it’s an idea.
Or just make the tank part of the final wave.
Better idea: Have the tank strapped to the top of an osprey that flies around the outside of the map, and when you kill it, it simply crashes somewhere outside the map. 
Mentioned earlier that I’d ported in a map from my HL2 mod to test the survival mode. Check it out running in BM’s engine!
Action shots. I threw a lot of NPCs at the map, so the framerate can get very stutter-happy, particularly at the start. This will be fixed in further iterations. At any rate, it’s a good stress test for the map.
Enemies on the map:
Headcrabs
Zombies (Scientist/Security Guard)
Vortigaunts
HECU (Commander, Grenadier, Grunt, Medic)
Bullsquid
Alien Grunts
Assassins (These are my favorite- the map has three stories so the assassins can flip and leap between all three tiers harassing the player. Highly unpredictable)
Whoops, some of the cell doors in shot 2 are visible in the editor but not in-game. I’ll have to backport some of the changes I make to my mod later. (Being careful to use HL2 textures so there aren’t any broken parts when I add it back in later)
So yeah, WIP.
Hey! I’ve just played your map!
I have obliterated 4 waves, then a crazy amount of assassins took me down!
It’s funny, if you have time you should work on it!
It reminded me orange maps in tf2
If you need a tester you can count on me!
A question:
in order to play it,shall I add the map name in maplist.txt?
why not just play TL2 or Counter strike?
I don’t see any reason why the developers should waste their time with this. Especially when they aren’t getting paid
As Parnifia tried to say in his awfully rude post before, It’s an idea for a community project. And if people is ready to develop and play it, why not ?
Hey guys, back! I am not done with the map yet, but I just want to let everyone know that I am still working on it. I am focusing more on the design of the map, focusing on things such as the location of first contact, layout, etc. It is taking time, but progress is steadily moving forward!
Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.