The final mission is divided (mostly) in four parts:
1. Killing Helis: use whatever weapons you have, except arrows (he can block them with his bracelets). Take advantage of the explosive barrels around the area. If some of his men are near him, use explosive weapons (they can be a pain in the ass, so decide if taking only Helis down, or the other enemies first). If you take too long, the arena will fall down and you will lose.
2. Defende the Ridge: After killing Helis, travel to the Rigde, pick up an oseram cannon, and shoot whatever is closer to you. Change sides if necessary.
3. After waking up, follow the path of destruction caused by the machines. Don't fight, just run. This is the easiest thing.
4. Destroy the Deathbringer guarding HADES: the final boss of the game, a buffed up Deathbringer.
Kinda dissapointing. Being buffed up means that:
- It has a lot more weapons than usual: machine guns, flamethrowers, grenade launchers, missile bursts, etc.
- You can't tear it's weapons, I think. I don't remember doing it on normal, but I tried it on the hardest difficulty and couldn't do it. So don't be surprised if you can't do it. However, you can destroy them.
- After a while HADES will summon corrupted machines, that will be a nuisance for the time being. While your comrades will take care of them, HADES can command them to attack you specifically. I recommend you to tie them with the ropecaster, since ignoring them will be troublesome, and attacking them will leave you defenseless against the Deathbringer long range weapons.
- It will be a timed battle. While this can give you anxiety, there's nothing to be worried about. The many times failed was because the machines killed me, not because I ran out of time. A lot of people really don't care for the timer.
- While is powered up, is still a Deathbringer. It will heat normally and take out the rods to cool it down. Use fire weapons to heat it faster, and destroy the cooling rods with high damaging arrows.
Once the Deathbringer is down, use your spear on HADES, and you will get the achievement.