Not nearly as daunting as it seems. You must indeed survive 100 waves of enemies in "SURVIVAL MODE", but not 100 in a row.
Here is a simple way to boost through this in around 5-8 hours.
1.) Pick a level you're familiar with. (I used PYRAMID. Click "Switch Map" until it returns to your chosen level.)
2.) The first wave of enemies is usually 12-18 of the gorilla like mummies, use your flashlight to make them burn brightly, then shoot or machete them as quickly as possible.
3.) Once it's announced that "THE VAULT IS OPEN" go to any trap that will kill you, stand in it's kill zone, then set it off. (the only one on PYRAMID guaranteed to kill you is the crushing walls in the hallway.) This will end your round and count as a wave completed. Time to wave completion is 1.5-2 minutes once you get in the rhythm of the level. Total with startup wait time & load time is about 3-5 minutes
4.) Rinse and repeat.
On the main menu, you can check your stats to see how many more waves you need.