I've just got into Magmoor Caverns for the first time. While I was going..

Yeah - don’t go into Magmoor Caverns just yet!

To get to the Burn Dome - you’ll need to go to the Arboretum, find and scan 4 Runic Symbols like you did in the Watery Hall to open a door. The first one is on a little island against a wall amidst the poisonous water (where the red weeds are). The second is located up a bit higher - use the platforms connected to the big tree in the center to get up to a small Morph Ball ledge. Get across it by blowing away the blocks with your Morph Ball Bombs- and after you pass another small ledge - accessible only by using your Morph Ball, you should find the second runic symbol. The third is above the small bridge made out of sticks on the wall and the last one is on the tree opposite to the hole that goes to the door ya need to open way up at the top of the room. Make your way through the Morph Ball hole, the door and the following hazardous corridors filled with Toxic Grass and Reaper Vines to find your first REAL boss, with Flaagrha. Get the health and the ammo in the crates, save the game and go kick his ass.

Look around the area and you’ll see 4 mirrors - in the beginning, only one mirror is active and shining light on Flaagrha. That light is the source of the plants’ power - that means that you must somehow stop that light from shining on the plant. First, stun Flaagrha - get into the vicinity of the active mirror and start pounding Flaagrha with your Power Beam until it is stunned. Once it is stunned, shoot the red button on the backside of the mirror; this will cause the mirror to deactivate and Flaagrha will take a blow and fall - paralyzed - to the floor. Now is your chance to damage it - your view will automatically show the vines guarding the tunnels that lead to Flaaghra recede the first time. Morph Ball into those tunnels and at the end you will find a Morph Ball Slot. Bomb it and you will hit Flaagrha and do some damage - after you damage it the first time, not only will the original mirror reset into its default position but ANOTHER mirror will activate, giving the monster more energy. The same strategy still applies; stun the beast, then quickly deactivate the mirrors. Make sure the thing is stunned BEFORE you attempt to deactivate the mirrors, otherwise Flaagrha will reach with a tentacle and reset the mirror itself (the bastard). After you damage it again, three total mirrors will become active. Hit it again and then one more time with all four mirrors functioning and you will kill the source of the poison that is plaguing the Chozo Ruins and the water.

After you defeat the plant-monster, grab the little ball, which is actually your Varia suit and watch the sweet clip!