My pick is Parace L'sia from Arcana Heart 2 and 3. She can 100% you from her spamming her supers, killing you off screen and will get you to rage. Here's the thing, once you lose to her 3 times in a row, her health is lowered. So with 3/4 left, she still isn't easy. After the 10th or 11th time dying in a row, her health drops to 1/4. She is still HARD! Her health slowly regens if you watch her health bar throughout the fight. She has a move where she charges her super meter to max!
Runner up is Ino from Guilty Gear XX Reload. All her moves give about 20-25% tension.
Omega Rugal and other SNK bosses don't look that bad now.
Any stories or achievements over beating a hard boss?