How to beat the hidden boss tips:
Make sure you get to at least level 27 before you attempt the hidden boss, make sure that Axl and X each have 4 battle chips, and that they are fully upgraded. You want high HP and SP (at least in the 900's otherwise the fight will be over quickly and you'll have to start over. Having the biggest healing tanks and a lot of them also help you. And make sure you never attempt to fight him on low health. His "Into The Void" attack does a lot of damage. So when you're at low health, use a healing tank. Having Airman as a chip is very good because it gives you an extra 100HP and Plantman is good as well because of his high healing techniques. Sometimes they don't work when you're facing him however.
I think this is one of if not the greatest MMX RPG's I've played so far. However once you're in the high level 20's, and you finish all the sidequests the game can get pretty easy and repetitive. Using different battlechips allows the game to still be fun but don't play it non-stop. It's more enjoyable if you take breaks from it every now and then. I like how there's achievements to get which gives you something to look forward to. Good luck and when Chapter 2 comes out I hope it's as good as this. There might be a few glitches in there but overall it's a great game.