A simple guide aimed to give information about reactor "abilities", so you won't be surprised easily. While it is natural to assume every reactor you encounter is a beefed up version of the previous ones, some of their abilities may surprise you. These ability names are made up by me; they are not official.

I hope this guide acts as a "heads up!" and help you on your journey.

-No defenses
Just blow it up. Other ones won't be so easy to deal with.

-Laser barrage
As soon as you initiate the battle, it'll engage laser defenses, scorching the room while spinning. Do not panic; use vectoral movement to get away from the lasers.

-Plasma spray
Keeps firing at you, with a faster rate of fire as it gets more and more damaged. Encircling is not recommended as there isn't much space, instead use the geometry as cover. A weapon with a rather good accuracy may be required, especially to save time.

-Laser Barrage
Same as Sublevel 1, as you initiate the battle, it'll engage laser defenses, scorching the room.

-Hazardous Environment
Lasers will cause explosion when they hit the lava pillars, so keep your distance to them.

-Turret Defense
A few turrets will be spawned to defend the core. Dealing with them first is recommended. They will remain in the reactor room after the core is destroyed.

-Laser Barrage
Same as Sublevel 1 and 3, however this time it is activated after core takes a moderate amount of damage.

-Turret Cluster
There are 9 turrets in the reactor room, 3 on each side, and 3 near the core itself. The ones near the core are shielded, but you can destroy the ones on the side. However the side turrets will respawn once the reactor battle is initiated, with shields this time. These turrets will force you to move constantly as you are taking shots at the core itself. Turrets will be destroyed as soon as the core goes down.

-Minefield
There are a lot of mines in the reactor room, clearing them out before engaging the reactor is recommended, as you will be on the move constantly.

-Seeker Drones
As the battle is initiated, a swarm of seeker drones will be launched at you. These drones will zap you if you stand still. Can be destroyed individually or as a whole with an AoE capable weapon. Unlike turrets, these will persist after the core is destroyed.

-Hard Shell
The reactor core is protected with a hard shell, which is composed of multiple pieces of armor. Each piece will absorb a certain amount of damage, so a secondary weapon with a rather large splash area & high damage can make short work of the individual pieces.

-Shield Generators
There will be two shield generators on each side of the room, they'll lit up like a lamp as soon as the core shields itself - which happens at the beginning of the battle. These need to be taken down before you can start working on the outer shell.

-Reinforcements
There will be constant reinforcements - lesser enemies will keep spawning as long as the reactor is still operational. Ignore them unless you really need to take out a few, but they'll be replaced after a rather short while.

-Turret Cluster
After the shield generators are taken down and the core takes some damage, multiple shielded turrets will spawn in the room, one on each wall, close to the the opposite side (ceiling/floor) of the shield generators. Be wary of their position, and try not to get too close to them while encircling the core.

-Seeker Drones
After the shield generators are taken down and the reactor takes moderate amount of damage, a swarm of seeker drones will be launched at you, like in Sublevel 4.

-Plasma spray
Starts firing at you like the reactor of Sublevel 2, but after suffering moderate damage.

A good weapon to deal with the core itself is recommended. Trying to destroy the core with a Smartgun for example, is not a smart choice (no pun intended), as you'll run out of ammo while dealing very low damage compared to pretty much any other alternative.

Game encourages (if not forces) you to stay mobile, so if you are having trouble getting used to the controls, reactor of Sublevel 1 is a good practice choice, as it only has laser barrage and that's it. You can practice avoiding laser while moving around on that reactor without sweating much.

And here is an extra information. Trichording (an old Descent term/slang) means that if you are moving on multiple axes, you'll have higher speed. If you are moving in 3 axes at the same time (up, left, and forward for example), at 100% trichording, you'll be moving at ~1.73x the speed (root of 3), at 50%, ~1.37x the speed. For 2 axes (forward and left for example), at 100% trichording it is ~1.41x (root of 2), and at 50% it is ~1.21x. No trichording means you'll always move at 1x. Thanks for reading, now go and find the exit.