Mr. X, Streets of Rage 2. Welcome to my SPOILER ALERT.
Beating Shiva is hard enough... he breaks through any of your combos of +3 hits, and likes to back-slam you all over the place. It's a pathway straight to hell on Mania difficulty.
But Mr. X? Not only does he move around all the time, not only does he have a seemingly infinity number of drones to help him fight you (emphasis on "seemingly..." he does run out after you beat about a million of them), but he also has a machine gun that he likes to shoot. All the time.
The normal strategy against him is that if you have Skate, you can just skate (run) past the stream of bullets if you time it right, and get by him. But if you're late at all, then he'll turn around and crack your skull with the butt of his gun. More baddies keep coming, so they can break your combo, team up, and it's hell from there.
Anyone else? Stand still and do the special attack- it doesn't take energy unless you hit someone, and it makes you invincible as the bullets pass. You know, unless he catches you in that .05 seconds while your char comes out of his move. Or if you're out of energy- can't do the attack.
Or if he gets bored and decides to run at you, swing his gun at your head, then stand over you and laugh, just to spite you.
SoR3? Time limit, bomb? Oh hell no. I hope you burn long and hard, Mr. X.