The quest: eliminate twenty-five invisible guys to accomplish the mission. I can only find twenty-four. Is the last one hiding some place difficult to find? Anybody else have this problem?

Heck yeah, I once had 8 minutes to find only 2 invisible guys and I still would run out of time. None of them are hiding any place difficult to find, you just really have to use your radar effectively. It may help if you kill the mechanical enemies first to get them off of your radar. Thats how I ended up beating it.

Good luck.

Are you referring to the big Samurai's?

Lol, yep. The ones you thought couldn't be killed.

You can use the sonic aura when it looks like your close to them on the radar this will make them visible and they'll make a run for it, or the blastsphere to kill them without seeing them must be level three though to kill with one hit.
If you finish the game once then start it again on ex mode for whichever difficulty you chose you can buy a psionic called deatomizer which will kill most enemys with one hit over quite a large area.

I used the Volcano to off most of them, after acquiring a part that enhances the barrel I did them in with one shot. The level's pretty tough tho, took me 3 tries to get it right and on the first one I was in the exact same predicament. I had 4 minutes to find 1 guy and I never found him. The toughest part is on the left side of the map, there are like 3 of them all bunded together and as soon as you kill one the other two bolt to different locations.

ha I used a Rocket launcher and i got Standerd Mode Ex and G Squad mode eX and my blastspear was lvl 5 and i BLASTED them all with 1 shot i wa sOoOoOo Close to getting Lvl 6 but lvl 5 bastspear was TOOO AWESOME! Well ill reply to this thread later im Soooo Tyred

I think I got to level 2 blasphere. I never used psionics much except for healing which was level 3 by the end of my first game. I personally just switched between the grenade launcher (Volcano) and the machine gun. The other guns were creative but not my style.