
I finally was able to unlock the last suit...and get an A rank on the last boss.
I'm fairly certain that you must beat level 6 with at least a B and then immediately beat the final boss with an A rank without continuing. I did this with the alpha leynos(#3) suit and beat the boss with L-20000 and MGX-10000 combo without getting hit.
I think the key is beating the stages 6-7 back to back without dying, and also the suit 3 is the only one I've used that produced the result of an A rank on stage 7.
Also the last suit is very interesting...Although it has the same weapon(wave lazer thing) as the boss on level 6, I don't think they are the same suit. Have to play again and look more closely.
Many thanks to you, wip3outguy7. I had pretty much given up on trying to unlock that last suit. You are the only person I have heard of to do so and your strategy proved invaluable...Enjoy Bulk Slash... thats one I have yet to play.