Played this in 2007 and still playing it in 2013, It's a great game and totally worth playing.
To answer any questions that may or may not be wandering around to he randomg amer who unearths this game, here are some helpful answers/tips.
1. before you leave the tutorial cave, take off Calisto's rifle and give it to Jin for a much nicer starting weapon than a crummy fist that does horrible damage.
2. Donna's last name is Drew, though you can find this out after some dialogue with the clothing store owner.
2.5. There is a secret area in the first jungle in the deeper area near the entrance to endless gorge, hidden as 2 patches of grass acting as a doorway. aside from a few decent enemies to grind on for the time being, the ultimate reward from the secret area is a chest containing a mediocre sword for Jin's Avenger MaW (if you happen to come here with his Luna MaW).
3. The derelict avenger boss in the scrapyard is basically a tank boss, so even at a decent level and decent hp, he won't go down easily, be sure to bring repair kits.
4. There is sadly no real easy way to find plasma rocks in lazlo mine, their placement is entirely random.
4.5. by the time you get to the grindworm boss, you should have billy's overdrive, megablast (learn at lv8 as with all overdrives), which makes the fight much easier.
5. you can pass the endless gorge after getting the plasma rocks in lazlo mines, aswell as getting Jin's new Avenger MaW.
5.5. There is actually a boss in Costa De Acero, but it's well hidden in a way. the boss in Mr. Chao, you fight him by angering him when you repeatedly mock his fried rice after buying it, you then engage in an MaW fight with him. generally if you have the full party he's not too tough. beating him gives you Jin's ultimate sword, which is a giant meat cleaver that IIRC deals multiple hits. you can also never buy fried rice from him again...
6. The sprout lord boss in iron jungle is hidden in an area that's entered from 2 patches of grass that are in the position of a path/direction near the end of the jungle, about 3-4 rooms away from the entrance to the beach north of iron jungle.
7. Gears and gyrobolts though they both augment HP, DO have a difference. the amount of hp you can augment to caps at around 250 for gears, so afterwards you have to switch to gyrobolts to get the in-game max hp of 500.
8. I've beaten Ariel Highwater (Serra Superion), so it IS possible to defeat her, but it takes max of all defensive traits to even survive against her deadly claw attacks. having your entire party with max hp (500) and their best armors (the highest armor in acero terminus for jin and billy, agility gear for callisto (drop from sprout lord boss in iron jungle)) is a must, aswell as repair kit E's for any sticky situation. As far as i know, overdrives abilities' effectiveness is based on level, so having a decent level (15+) is a must. Keeping callisto on Multifix is important for party healing. if you dont have a good weapon for Jin or Billy, using their overdrives works well.
in case you cant scan during the fight:
SERRA SUPERION
HP: 5000
Armor: 10
Accuracy: 60
Reflex: 35
Credits: 5000
EXP: 3000
Successfully defeating this normally unbeatable boss nets you a strange gun called rolling launcher. IIRC only Billy can use it. you attack by lobbing 4 grenades at random enemies While its arguably powerful, it acts more as a spread shot for dealing with groups(or just adding some RANDOM into your fights). For strictly power, the Y&S Suppressor is mainly better, even though by the time you get it (which is after beating the pirate boss in tidal caves), it's use is. well, useless, seeing as you've probably beaten every boss before even getting it.
ANYWAY I hope this small guide was at all useful for anyone who still plays this and hope they can enjoy it better as a result!