I'll duplicate some stuff from Dalshim, but some of his points are not so important, and some are missing.
- Play on Normal
- Stick together
- It's better to have a non-full team (i.e. less than 4) than to take bad players into it. Three players is OK.
- If you want to take Hornet Barrage (or, even worse, Smart Bomb), better take Electric Charged Armor (it's very useful, not only against parasites which may ruin your session) or Freeze Grenades
- The more medics the better (the best loadout for medic would be Flamethrower + Healgun).
You shouldn't be a peace medic; such characters are just too vulnerable!
- Use Jaeger with Vindicator to kill the aliens efficiently! It's a complete disgrace to take Flamethrower if you are an officer.
- Tech could take a sentry (a simple one, not the "sophisticated" but less useful ones) for the missions where you stay in some place for a long time, and Officers can carry Incendiary Mines which are even better.
- In my personal opinion, Special Weapons class is mostly useless. Better take one more medic.
As usual in Serious Sam 3, the area in question is in serious plain sight. Acute players will have noticed this small green blinking item in The Power of The Underworld, roughly 400 kills into this serious level (after the serious infinite rocket storage).
It's seriously right under the crosshair. But how the hell would you get up there?
Climb on this seriously damaged wall further ahead, opposite the construction area leading to the next serious part of the map.
From there you'll see serious "secret keys" blinking in serious purple. Go back to the where that green item is located - the serious gates will now open for you.
You'll find a seriously long ladder on your left, before heading through the second gate. Climb it up, and, sure enough...
It's no crash to desktop, but it's a serious sight to behold, plus you get to have a peek on the following serious area. A very good peek I might add, since the green blinking item just around the corner is a serious sniper rifle. Unfortunately, it only comes with 5 serious shots and it appears it only can hold 10 anyway.
Remember to take the ladders on your way down, or you'll simply crash under your serious weight and suffer a seriously humiliating death. You can simply start taking the stairs with E, then jump off them and press E just before reaching the ground serious fast.
Best Answer
As explained in the Achievement Guide by Cassel, the combination can be found in another game, Device 6 (unfortunately only available on iOS), and is:
(where L means "left owl" and R means "right owl")