This guide shows you excactly what to do from the beginning till the end.
There is also a guide below, which will help you to pass the final arena fight very easily.
In Addition, there is another video for the guys who can't watch the video above, below (In Germany, it's not available). Unfortunately, this video does not show the easiest way to get the achievement. Nevertheless, the easiest guide is posted below. (Comments forced me deleting the video)
Tipps for the Arena Fight:
The Arena consists out of 4 enemy types:
1.Kamikazes: Just look around and kill them before they hit you.
2."Flying dogs": Just pass the arena like a circle and always look back and shoot the enemies who are coming too close. After some time, there will be a bunch of enemies in the middle, which makes it really easy to kill them.
Okay now, things are getting tricky:
3. Bulls: Right after the "Flying dogs" Part, there will be a health package and some armor in the middle. Pick it up except the ammunition. Go straight to the left or right corner (From the Entrance) and do a rocket jump up on the wall. I recommend these corners, because they furthermost of the rocket robots (See enemy type 4). A few seconds later, the bulls will respawn. Now its really easy to kill them. Just use the grenade Launcher for them. If they are too close to the wall, i would recommend using the laser rifle.
4. "Rocket robots" and Kleers: After the bulls, 2 red robots are spawning in front of the exit of the arena with some kleers too. This part is the hardest. Just focus on the robots and try to avoid getting hit by the bullets of the Kleers and the Rockets. I would recommend to use the grenade launcher again to kill the kleers. After you killed the next 2 enemie waves, Sam will be saying "Yehaaaa", which means that the door is open.
-Select infinite ammo (You will still get the achievement). Just press Y in the menu, where you have to select the difficulty.
-On each Wall in the arena, there is a little place, where you can't get hit by the enemies. You might want to use it for the "Red Robots".
-Instead of doing the first Rocket Jump to get out of the map, I would rather recommend to do it on the right corner, which you can see, when you come up the ramp from the beginning. There is also an armour on the left in front of the wall.
If you follow the way of the video below, you will have to kill too many enemies, which makes it havier.
-If you die, don't load any savegames, even not the one which was created right from the beginning. Quit the game and restart it manually.
-Turn the auto-aiming with notation in the menues on.
-For the first rocket jump, you need more than 50 healtpoints.
You ve got problems surving the "Spawning Part"? Here is the guide:
When spawning, there are 3 enemies in front of you (downside), one above them, on a wall, another one on the right and 2 "Greenys" on the left and right tower.
At first, kill the 3 guys in front of you with the laser rifle. Now keep jumping and shoot the 2 bullets of the "Greenys" (Its not that bad if one hits you). After that concentrate on the two enemies, which are on the wall. They might jump in the quarter where you respawned. So now its only you against the 2 "Greeney". Get into the left corner from where you spawned. There, the left "Greeny" won't see you and will not shoot at you. Just shoot the right guy and watch out, that the green bullets won't hit you. After that kill the left guy. When you get up the ramp, now, there will be a Kleer spawning in front of you. I would recommend using the rocket for him. On the top, there are also two little guys in front of you and on the right. Just kill them, pick up the armour on the left and do the rocket Jump where I wrote. After you killed all 3 of them, watch out for a "Greeny" who is on the very back of the level. You can't see him very well, because he is covered by the palms. He will keep shooting with green bullets at you so just kill him. With Auto-aiming with notation turned on, you will simply see him.
I hope this will help you to get the achievement very fast. Just stay concentrated and you will be able to get the Achievement even at its first try!
Please accept my apologies regarding to my bad english.