Defeat Shin Seseri at level 5*5 or over (Arrange mode)
First of all, you'll need to set the difficulty for a couple at 5*5 (both characters) in the option menu before starting the game. There is two ways to do this:
The easy way: Watch an All-Clear Replay (from the begining to the end, you can fast forward) from the leaderboard at 5 * 5 or higher, both levels need to be at 5 at least (i.e An All-Clear Replay Level 45 means that only one character is level up, so it will not work).
The hard way: 1CC (1 clear continue or, in other words, complete the game without using a continue) the game five times at player 1 and five times at player 2 using the same couple. Each time you beat the game without continue, you gain a level for your primary character only.
After this, you'll need to meet Shin Seseri, the true boss of stage 5. Before ever face her, you will have to make your way starting from stage 1 to the end of stage 5 without losing a life and reach level 3 kakusei overmode. To enter in kakusei overmode in Xbox 360/Arcade mode, you have to activate kakusei with no green gem in your counter and wait that the clock/circle around your character is full and disappear. If you die or return in normal mode, you will still have your level. However, in Arrange Mode, each time you switch between different kakusei mode, you lose level.
My advice would be to pick Tateha (there is plenty of all-clear replay at high level in the leaderboard), she have a very strong turbo shot in kakusei mode that auto-target enemies and cancel every bullets near and in front of you that make impossible to die. Again, the achievement is still hard, mostly because you will have not enough gold to beat a boss in kakusei overmode or you have too much green gems that would take forever to deplete before entering in overmode. Stage 1 and 2 are easy but in the other stages, you will meet a bunch of annoying enemies and long boss fight.
Be sure to have more than 900 golds in your counter and less than 150 green gems (use them as soon as possible to enter in overmode) before each boss battle from stage 3. If you not sure that you can manage the bullet pattern in the screen, press RT fast (without holding) to use a small amount of energy (guard barrier), the only time you will be able to replenish your guard barrier is at the end of stage 3, make good use of it. Also, if you think you will have not enough gold to beat the boss in overmode, return in normal just before destroying a boss part to pick some green gems and return right after in kakusei (normal mode with green gems) to give him the final blow.
One last thing, like I said before, you need to be at overmode level 3. At the begining of stage 5, you will probably be at level 1. To rank up at level 3, max out your gold counter before the middle of stage 5 where you got the power-up. As soon as you get the power-up, enter in overmode until the boss, you will have enough gold and, from here, just beat the boss even if you die a lot (you have already reach the true form). Achievement unlocked.