well I eventually completed the game aswell....
as for the endboss:
[sp]I used the twirl shoot thingy (X + O + R1) to keep the smaller scorpions away and damage him in the process. Then for his last bar, I simply used dark jack to take half of his energy away and used the rest amunition to blast him away. took me a few tries, but eventually got through without serious damage.[/sp]
All in all, great game, though I wish they would have given us 70% adventuring instead of 70% city. Meh.
