Welcome to the hardest achievement in all the serious sam games.
When i achieved this i did it in co-op with one friend, we had previously ran through the games twice in the beast mode which gave us a knowledge of the game, and it took us 2 atempts to beat the coin-op.
The only thing i can suggest is know the game very well, but this is the main reason i am writing this as the game screwed me and my friend over.
Also enable auto cycle maps, private session, start on map one and no unfortunatly you do not have infinite ammo, and if you all die, start from the begining again
Okay, the last level is by far the most of the unfair level ever made. I suggested your practice this level a couple of time before attempting the whole game. We played through the level normally to get the extra lives, but once we had left the end of the long corridor and reach the place with 2x +50 armor and 2x +50 health just outside the cathedral, you are supposed to kill a lot of enemys before the door to the final boss will open. Instead rocket jump over the wall (practice this first) and skip that section, (follow the video for cathedral king achievement on that solution) and fight the boss, hide where he does and kill him co-op however if someone fails the rocket jump do NOT spawn in, just wait for the boss to die HOWEVER here is the warning, the game glitched on me both times doing this achievement, during the final boss fight mid battle the game teleported me first attempt and it spawned me from one side of the boss arena to the other and i died instantly and we failed due to trying to survive the waves of enemies before the boss. The second atempt my friend hid round the corner of the cathedral, which works as a great pick-off point of killing the enemies before the boss but he failed the jump however i suceeded and was fighting the boss like in the video. However when the boss had about 1/4 health left the game teleported me out of the boss arena and spawned me back at the ammo/health point. When this happened due to the hundreds of enemies spawned in i died 4 times in a row and managed to get round the corner where my friend was, we had no lives, the door to the boss wouldnt open and we havd no ammo and there was about 300 enemies in the way. what we noticed just before we gave up hope that the boss was slowly still loosing health. What i think was happening was the boss spawned in a big red mech and when the boss spawned closed the wall we were at the red mech atmpted to shot us but kept hitting the boss. When we noticed this we started to pick off the enemys between us and the boss gate and we had to use pistols, however the boss died after about 1 hour of this, thankfully when the boss died all enemies in the room with him died also but there was still alot of enemies between us and the boss room. It took us a further 20 minutes but im not sure if this was luck or not but this could help you when you attempted it as it could happen to you if you get forced out the room.
To sum up the whole game is easy but this last level is totaly unfair after the long corridor It takes skill, luck and hope that the game doesnt spawn you mid battle to your death.