While searching for the wooden pipes to get the Mon, I enter the Achondar place, and while I was able to get the pipe and get out of the place, I tried to kill the thing. I wasted ALL my ammo (about 200) for my weapon (the Hawk-thingy lvl 2, the one that need to charge and shoot a... strange colored shot), but it didn't finish the bastard.
I tried aiming for the head, and the shot looks like it hit (it occurs an explosion, and his head kinda change direction), but nothing happens. In a FAQ from gamefaqs, it says that it takes 10 shots from the weapon to kill the thing.

Thanks for the help, and sorry for any bad english.

EDIT: I'm using the High Res Patch

I just encountered and killed the Achondar! In case you still want to kill that beast, here is how you do it:

*** SPOILERS AHEAD ***

Fire a grenade (weapon no. 5) far away from your actual position (e.g. behind the Achondar) to distract it. You mustn't move or fire to make it turn around. Quickly change to the fast-firing gun (weapon no. 2) and give him some bullets right on the back on his head.
Repeat this several times and you should be able to defeat it.

I'm not so sure, if that turning-around part is really needed. Some guy once said, that the Achondar was only vulnerable on the back of his head. I never tried direct fire though.
I also upgraded "weapon no. 2" once, no idea, if it makes a huge difference.
Got the high-res-patch installed, too, so hopefully it will also work for you.

Happy hunting!

Yes, the Achondar can be killed. Once you enter the arena, the entrance closes and you are stuck until you kill the beast. Run to the Northeast side of the arena, there you will see a tree stump you can hide behind without taking any damage. You will need the grenade launcher which is the LN-DUO 500. Back up from the stump slowly while facing the beast and you can still remain under cover while lobbing grenades at it. It will take 8 grenade launcher shots and a few quick shots from your UZI-SH1 to finish him off. Use the button-on the floor around the lava pit to escape.