Sabaki Ryuuken (Azcal)-He can counter anything you throw at him including desperation moves, he has a move similar to the Raging Demon and depending on how much HP you have it's instant death.
Saiki (Werewood)-I haven't played KOF XIII but i doubt the source game is super cheap like this one.
Ikaruga's characters-Almost all of them have super cheap AI that revolves around comboing the shit out of you nonstop like it's Killer Instinct. X___X
Muteki's characters-His Guilty Gear conversions are crazy to fight against. His I-No and Ky Kiske suffer from piss easy A.I though.