Has anyone been able to defeat the giant monster, Achondar? If so, please let me know how you did it.
I've read some walkthroughs and followed their instructions. I used up all my Hawk-MX ammo (about 180 rounds) on the monster's head and most of my UZI ammo. The thing just won't die!
Is there a glitch/bug or something?

You need to use the smoke bombs from the old hunter's (sorry, the name escapes me) custom-made gun first. Once the beast has been blasted with a few smoke bombs from that weapon, it'll be temporarily stunned. Once it's stunned, shoot it in the head with the biggest gun you have. I'd recommend the plasma-laser weapon over the Hawk.

I also had trouble defeating it, since apparently I couldn't damage it most of the time. After a lot of reloading savegames and trying different weapons and angles, I noticed that seemingly I had better luck defeating it from the back side of the arena, opposite to the entrance gate. There it would usually, but not always, die from a lot of HAWK-MK8 rounds. Though, how many shots it took varied. Sometimes it only took 10, sometimes 70.
If it still won't work, though, don't worry. You don't need to kill it and the only reward you get for doing it is that you can brag about it to Kyuran :)
You are mistaking the Achondar for the Gorgor. The Gorgor is the monster on the island in Okasankaar, the Achondar the giant fire-breathing snake monster in Okaar :)

Oh. Oops.
Yeah, what Syrion said is right. Just aim for the head and it'll die eventually.