Misery is easy if you're an okay shot with the spur. Doctor 1 is easy once you learn how to jump in the center of the two crystals. Doctor 2 is like Toroko - Some people have a hard time dealing with his jumping, I particularly don't.
The end fight is usually hard because of Misery and Toroko. I suggest getting rid of Toroko first and then Misery, but make sure you use Misery to fill your health if it's low before getting rid of her. Dodging the Core is simple, since he only shoots on the floor or in the center.
The first part of Hell is just learning how to use your Booster, not many tips beyond filling your rocket launcher with all the experience in the room.
The second room gets easier once you realize that blocks stop hurting you once they hit the ground. Using this, you can anticipate a collision and delay a bit, letting you pass through those huge blocks harmlessly instead of trying to jump over them. My advice is to not stop instead of taking it slow and accurately.
The long hallway has a lot of places to get health, and I suggest getting them all. Stop at Bute pits to kill them before they screw you over. Use Curly's Nemesis for long shots, and make sure not to miss the wallbomb things lest you waste time waiting for them after you've finished fighting. DO NOT MISS THE MISSLES IN THE TOP OF THE AREA BEFORE HEAVY PRESS.
Heavy Press is easy once you learn his pattern. Basically equip the Spur, aim up, and let loose. Keep an eye out for the rolling eyes, and every so often shoot to the side to kill the butes that spawn. One thing you need to watch intently is his lightning gun. You see it glow, run and take care of Butes for a second or two.
Ballos first form should not hit you at all. Pound him with a fully charged Spur as soon as he puts his hand out, and then shoot him as you jump over his charge. Shoot up and run under him as he zooms back, and then pound him with Curly's Nemesis when he lands, and hop over the shockwaves. Rines, repeat. If he's not dead by the time he starts to fly, get under him and dump mad Spur up his butt until the lightning starts, then just run back and forth.
His second form is a bit trickier but still easy. Make sure to run once you see the target form around you. The kicker here is if you can get a fully charged Spur shot into the sweet spot just below the middle of his eyes. If you do, it will go THROUGH him and do way more damage than normal. Basically it's a game of running from him(Or boosting under him if he gets too close) and aiming charged shots at his eyes.
The third form you just need to run around the room with Ballos. Run away from him and put yourself in one of the gaps between the rotating eyes while he's on the bottom, all this while shooting to the sides. On the top, stay near his front and shoot while aiming up and running with him - make sure not to run too far or you'll be stuck under him.
Fourth form? This is where your charged up rockets come in hand. Equip the Spur and wait for the spikes to reach just a little left of the middle of the room, and then jump on them - they only do two damage. Then, while you're invincible, fly up into the eye on the left and keep walking into it - he can't hit you while you're there. Equip the rockets and his life should go down in less than two seconds.
I get an average of 7 minutes with this strategy, and I thank several speedrun videos for teaching me the different techniques. Although most of it's obvious, following a guide will probably help.
including having already used the Life Pot.
That also works with his jumping form, but you have to be careful not to end up in the bats.