People seem to have the worst time with the Torizo, but I breezed right past him. Probably owe it to all those misspent hours of my youth Space Jumping all over Super Metroid looking for hidden stuff.
Anyway, Tester was the one that gave me fits initially. Killed me three times before I came up with a decent strategy.
Stay on the ground as much as possible throughout the fight. At first he'll come down to you doing that buzzsaw attack that he stops doing forever very shortly into the fight, but eventually he'll tend to sit way up and rain projectiles down on you. In between attacks, space jump up a bit and shoot him a little to bait him back down. Use missiles to take the shields down, then regular beam shots to damage the core and destroy the guns (temporarily). The guns replenish after a short time, but the shields don't appear to regenerate at all.
You can shoot into the core after downing just one shield, but I found it easier to focus on trashing all the shields first then exploiting his positioning for one of his attacks to shred the core (see below).
For each projectile type:
1) For the orange fireballs, just dodge them all while on the ground, then jump up and try to bait him back down as soon as they end.
2) For the pink wave beam thing, stand either to the far right or far left corner while on the ground until it ends.
3) For missiles, switch to the beam and shoot as many down as possible. They drop health pickups if shot, but give you nothing if they hit you, a wall or just blow up on their own.
4) The green laser is the only time you should go upward in pursuit of Tester. Just follow it clockwise to avoid getting hit and take shots at him when available. It rotates four or five times before disappearing.
5) The blue splitting bouncy things are the big opportunity to pump shots into his core once the shields are down. When they begin, go wait in the righthand corner. Eventually he'll fly down and park just above you, in perfect position to raise your beam up with the L trigger and pump continual shots into the core (if the shields are down) Find The Ice Beam
Technically, yes, there are two visual clues on the screen where the ice beam is hidden, but they're dark and subtle and easy to miss, and on a screen that doesn't look conducive to bombing. Plus the game just gave you all these new powers and teleporters back to old areas, so it's very reasonable to assume that whatever you need to advance is hidden somewhere else and go wandering around fruitlessly for an hour.
Anyway, here's the location, thanks to some anon. Don't feel bad. Cheese The First Omega Metroid To Death
Here's a slow, boring and cheesy but almost-surefire way to safely kill the first Omega Metroid if you're running low on health or missiles or something and don't wanna have to re-do all that crap from the last save point on.
While you're standing up in the entrance where you first came in, you're completely safe if you're up on the ledge just before you exit the screen (if you're on the little ledge just beneath it, fire breath can still hit you and knock you off the screen). Carefully stand up there and wait for Omega to run into the rock formation in front of you, he'll then turn around and jump in the other direction at least once, if not twice. Drop down on foot when he turns around (if there's space) and do a couple of Space Jumps with the L bumper held down to angle your gun down and fire in midair to hit him in his back with a missile (in the green vulnerability slat that opens briefly when he lands). When he turns back toward you, retreat to your hidey hole and repeat. Practice with regular missiles until you land a couple hits and get a feel for it, then switch to super missiles to speed the process up.
I dunno exactly how many missiles it takes but for me it was like 5 regular missiles then 3 super missiles to kill him, all in his back.
Note: turns out you can actually do this to all the upcoming ones too. Just hide in the entry ledge in morph ball form. Given the shit hitboxes it might actually be the only viable strategy?