Strikers are the "summons" that you can call on by pressing the Left Bumper during battle. These consume Gut Points (the same points that power your character's special attack), so if you are attempting to summon one and they don't come, that could be the cause.
There are four strikers in this game. Knives Chau, Mr. Chau, Mrs. Chau, and Nega-Knives. Each of the initial four characters starts with the ability to summon Knives, while the secret character summons Nega-Knives instead. Mr. and Mrs. Chau are unlocked as strikers by finding and defeating Mr. Chau on the map screen. Only the character that beats Mr. Chau will unlock the strikers, but the attacks are the same regardless of who calls them. If a character has unlocked Mr. and Mrs. Chau, you will not be able to control who is summoned. Mr. Chau will sometimes come out instead of Knives, and Mrs. Chau is rarer still.
Mr. Chau begins to appear randomly on the Map once you beat the Tood Ingram. You will recognize him as a character icon in black, standing on any of the available levels. If you enter a level he is not in, Mr. Chau will disappear and you will have to wait for him to reappear. If you go to the spot Mr. Chau is standing on, you will automatically begin the fight.
Unlike other levels in this game, if you lose a life while fighting Mr. Chau you will be sent back to the menu and Mr. Chau will disappear temporarily. But don't worry, he will continue to randomly appear occasionally after you clear an area. It is typically recommended to play "The Main Stage", as it is fairly short and also gives you an extra life for your trouble.
You will not be able to control which striker appears once you have unlocked Mr. and Mrs. Chau. Mr. Chau typically will show up at least once or twice every 10 times you use a striker. Mrs. Chau appeared for me every 15-20 times I summoned a striker.
There are some discussions that this could be a glitched achievement, and there's a series of steps that those who have unlocked it have taken to avoid possible glitches.
1) Make sure you unlocked Mr. and Mrs. Chau as strikers before you beat the game with any character.
2) Make sure that you have successfully summoned both Mr. and Mrs. Chau as strikers before you beat the game with any character.
3) Make sure you summon Knives with each playable character. Mr. and Mrs. Chau only need to be summoned by the character that first unlocked them.
4) Make sure the striker actually hits an enemy with the attack. You will be able to tell this has worked as the enemies will take damage or react to the technique. One that frequently causes confusion is Kim's. Knives causes damage as she's leaving after the kiss. So make sure that one hits as well.
5) Make sure you have unlocked the 5th playable character by beating story mode with all 4 original characters. His striker is unique and counts towards the achievement.
With these tips in mind, the hardest part becomes beating Mr. Chau. So good luck!
"If you're coming here because the chievo glitched on you, here is a guide on how to "unglitch" it and get another chance at doing it: http://www.xbox360achievements.org/forum/showthread.php?t=25...