This should be an easy fight. Stand in the center until the wolves run along, then move out of the way and take a few shots at them. It'll take a couple rounds to win but you shouldn't lose any lives if you're patient. The lava balls are slow and easy to avoid, they shouldn't get in your way much.
You can damage Riva with a direct hit to the body, her legs will not take damage. You can attack from above or below, I find above to be easier.
There are a couple spots you can stand where you won't get hit. Pick one and just blast away for a minute or two. This has to be the easiest boss in the game.
This is the first difficult boss in the game. A fast character will fare better than a slow one with higher attack. When she's in tornado form all you can do is dodge.
When she's done spinning take as many shots at her as you can. It'll take a while but if you keep dodging you'll come away unscathed.
Mynos is extremely slow, if he manages to hit you then maybe you should reconsider video games as a hobby. The spheres rolling around the room may get you a couple of times though. Just stay as far away from him as possible and shoot when he does his hammer thing.
Like Bel, when he's spinning there's nothing you can do but dodge him.
Keep your distance and shoot him when he's done. Take out the bats if a good chance arises but primarily focus on the vampire.
When he transforms things begin to get difficult. It's tough to avoid him at this point. Alex or BE are good here for their shield magic. Keep shooting at him every chance you get. He won't last long in this form but can dish out a bunch of damage before you finish him.
This is another fight where a fast character is preferable. He spins all over the screen so keep running and shooting.
You have to score direct hits to the blue spot on his head to do damage. The only way to accomplish that is by lining up in the center and shooting. However, this also makes you a sitting duck when he starts firing bubbles. When that happens clear out of the way and start again when he's done.
OK, I take back what I said about the tree being the easiest boss in the game - this is the easiest boss in the game, in any game I suppose. He never attacks, just stands there waiting to die. The annoying part is the dialog after every hit. Eventually you'll take him out.
He can only be damaged with a hit to the nose. Sometimes you can hit him diagonally but head-on is the most reliable. When he pops up take a few shots then get out of the way from his attack.
His first form isn't too rough, the ol' dodge and shoot will work.
When he splits into smaller forms then things get chaotic. Keep moving and shoot the second he appears because he won't be there long. Efrem or Fina are good here because their dark magic will take out all the annoying purple worms.
This is another battle you'll win through patience. You can only hit him from behind. Trying to run behind him, even with a fast character, is basically impossible. Instead position yourself to be able to hit him after he runs into a wall and turns around.
This is the toughest boss in the game so far. In the first phase you can only hit it when the shields are down. Although you have more time to hit them from above or below, you should be able to get more hits in from the side. Stay along one edge of the screen and be ready to attack when they head toward you.
The second phase is very difficult to do without losing a couple lives. The best strategy I've found is to box yourself in a corner and hold down the fire button. Those little blobs won't hit you as often but you'll still take some serious damage. Don't worry about aiming, the boss will cross your path plenty of times.
This is another boss where a fast character has an advantage. You need to both dodge the falling rocks and keep a safe distance from him to avoid his flame attack. Keep moving and fire a few shots his way when you can.
After defeating Karn you'll receive the orb which allows you to upgrade all the initial characters.
You have to hit him in the eye to cause damage. He moves slowly so you'll be able to make direct shots easily.
Like any other centipede boss in a video game you can only damage him with a shot to the head. One strategy is to get in a corner so he comes at you head-first. After a couple hits his body scatters and re-forms so you don't have to worry about getting trapped there.
If you max out your attack power you can wedge yourself into a corner and take him out before he even touches you.
Trial Valley - Face
When he's circling around just keep out of his way and shoot when you can. When he stops to do the weird hair twirl attack make sure you're facing him straight on from above, below, left, or right. Keep shooting at him while he's doing this attack. The waves he fires only go diagonally so as long as you're not in the path you won't take any damage.
Trial Valley - Rock Creature
Shoot him in the midsection to cause damage. He'll spawn two protective orbs that will make him a little more difficult to hit. Unless you're playing with an incredibly slow character you should be able to dodge him easily.
His light sphere attack is tougher to avoid. Initially these spheres form a wide circle around him and spin quickly. During this phase get as far away as you can. After they stop spinning they all launch directly at you, a fast character is preferable here.
For this battle, a character with high attack and high hit points is preferable because you'll get hit a ton and he absorbs a lot of damage before falling.
In the first phase of his attack get to the bottom of the screen and try to hit him in the mouth.
When he sticks out his tongue, move all the way over to one side and shoot it. Stay toward the top because it retracts as it gets hit.
Phades will teleport around the room. When we appears you have a few seconds before he attacks, use it to hit him a 2-3 times.
When his lightning attack is heading down the screen position yourself to avoid it. The shape is always the same so it's not hard to do.
After a few hits, Phades will escape and send Shad your way.
When Shad is flashing red your attacks will be reflected. You can try to learn the timing of the flashing or just get all the way to one side of the room so you have time to dodge any reflected attacks.
Throughout the battle he will disappear and materialize where you're standing. When that happens run over to the other side and resume firing shots. With some patience, this isn't a tough battle.
Gil is fast but not very powerful. Get him to come at you in a straight line and shoot through his attack to hit him. Once he gets close clear out of the way and repeat.
Just like in the original, you have to hit the gem on his forehead to cause damage. If you get into one of the top corners you'll have a clear shot and his hands will rarely get you. You will, however, take a ton of damage if he touches you so move out of the way when he starts heading your direction.
It is possible to defeat him without losing any lives but it's tough.
Initially the fight isn't too bad. He'll launch an attack where rocks spin around him for a few seconds, unless he's in the dead center of the screen you should have no problem keeping away from it.
After you destroy his body the fight becomes difficult. He moves around quickly and drops a powerful attack on you. There's not a lot you can do to avoid him so just keep moving and firing.
The first phase is all about survival. Destroy the blue flames while waiting for a path to open up. After a few stomps you'll be able to reach the exit.
Shooting at him diagonally is your best bet. Find a spot and keep blasting him in the head. Even with a powerful character it will take a ton of hits to defeat him.
Like any good boss he comes back for another round. Same strategy as before, find a good spot and attack diagonally.
He'll start losing limbs, once he's down to just a torso the fight is almost over.