round 3 Arenas are always a problem, especially with the mechaknight mob at the end. I started doing RT runs because there pretty easy with my Iceburst.
At first, i was going to get a catalyzer, but i saw how good Pulsars are. Whats more usefull?
Catalyzer vs. Pulsar with Arenas
i use a pulsar (polaris) and it's pretty easy to shoot them.
shoot once, activate their shield, shoot the other 2 shots (depends on how for you are from them but it's not that hard once you know the timing of the shielding) does take some time tough (oh and try to keep them in a pack, if the front shields and others walk in front of them they will shield and cancel out the damage again)
Fire bombs are optimal for the last wave in an arena. Mechaknights shield when you shoot, so killing them with a gun, while possible, is inefficient. Get a fiery vaporizer and work it up to Ash of Agni. You'll never find the mechaknights too difficult again.
For the 12-puppy wave, though, you can't go wrong with a pulsar (especially the kilowatt pulsar line). For polyps, the toxic catalyzer would be better because it is shadow damage.
As an alternative to Fire bombs, Shock bombs are optimal too. With the number of Mechaknights packed, I am not even sure the dps is lower, and the shock will slow them a lot, making them easier to kite.
Arenas are packed with lots of mobs, so really bombs are optimal there (but you need a sidearm to deal with puppies).
As for the original question : Pulsar is good, you can fire in the pack a lot, and eventually some hits will lands, even on Mechknights. It's also the gun of choice against any puppy, due to expanded shots which can interrupt 100% of the puppy shots (if you use it correctly).
Catalyzer, on the other hand, suffer from its slow bullets, making it hard to use, and elemental damage is often better than shadow damage (especially for RT). The charge, while flashy, is hard to land on fast mobs too.
A knight with a graviton and another with a voltaic tempest = guaranteed destruction!
On the guns, I think OP meant to shoot some catalyzer charges into the pack and then detonated them for widespread damage.
It could work , but sword charging should be easier. Or Quicker.
Get a fiery vaporizer or static capacitor for the final wave and a pulsar for the gunpuppies.
I have yet to use a catalyzer, but I feel like in crowded times like the final wave of the third round, you wouldn't have enough time to charge up your shots. I do have a Polaris, and it is rather useful. It pretty much shocks gun puppies and other things into a stupor, and properly aimed explosions can give you some crowd control. I mainly use swords though, so I'm not sure how good of an opinion mine is. I do hope it will help you make a decision though.
Your Iceburst should be more than sufficient for dealing with the mechaknight swarms. Just stick with using the charge attacks, and circle the arena. You should be able to do one charge per corner and not take any hits yourself, going in a roughly cloverleaf-shaped pattern.
However, of the two choices given, I'd go with the pulsar. It'll at least be useful for dealing with the 8-turret wave. Keep in mind that T2 mechas will shield when shot at, so a gun isn't really the optimal choice for going up against them.