Down, up, attack is a great move for survival and score (Nick Fury's has a longer range). Hitting something with the tip of the move gives you 1500 points (in addition to the point value of the enemy or object). If you're patient, you can leech Pretty Boy robots for a ton of points. You can cancel the move into attack + jump at any time.
With your pistol drawn, face away from an enemy, then turn and fire at the same time. If the shot doesn't knock the enemy down, you'll get 1000 points instead of the usual 100.
If you beat the bonus stage with twenty shots (one per barrel), you'll get 640,000 points. If you waste even a single bullet, you'll get 240,000.
If you're facing one or more knife guys, you can simply punch in place. They'll walk or roll into your punch range before trying to slash. You'll also block knives that can add thousands of points to your end-of-stage tally. If you end a basic combo with a kick, a thrown knife might sneak in; I recommend holding down so you end with a forward throw.
Take food containers to the first, fifth, and sixth bosses (there's a meat container in the fourth boss' room). Kill the boss while the food is on-screen, then get it for an easy perfect bonus.
Scully: Take the barrel at the start of the stage all the way to the boss fight.
Guardroid 2: After killing three ninjas at the end of the stage, lift the top oil drum. Don't fight the ninjas on the drum's horizontal plane lest they accidentally destroy it.
Kingpin: There's a big fight at the end of the final hallway (I burn three grenades here). Kill the last enemy while the bottom box is on-screen, then take it.
Easy Kingpin strategy:
1. Go down and throw your box at the two mobsters. To hit both at once, wait a bit before throwing the box.
2. Vertical dash kick or grenade the Kingpin, knocking him down. (Some moves, like attack + jump, won't keep him knocked down as long.)
3. Lift his downed body and throw him. Do a running jump onto his chest and throw him again; repeat until he dies.
1. When he's lying down, you need to be near his chest to lift him. It's not enough to stand over his head. Also, you can't lift him shortly before he starts to get up.
2. Sometimes, a mobster and the Kingpin will be knocked down in roughly the same area. If you accidentally lift a mobster, just throw him at the risen Kingpin and continue the loop.
3. Never knock the Kingpin into a wall (he'll get up almost immediately).