Being shadow, biohazard may do no damage at all on final phase (but will work on first one!)

Edit : valuable Neutralizer's user first hand experience below, so my answer is more or less bad.

Never saw a Neutralizer at all, but still, it's easy to extrapolate from experience :
- normal damage means the damage is not going to be good. You can make it work on Zombies because they are so slow, but you are better off with an elemental weapon. Trojans are going to be a pain because of Neutralizer slow bullets (I tried with a Polaris, which have about same speed bullets, and it's close to impossible unless you go close his back, and if you do that, you should just take a sword)
- no, normal shots of Catalyzer don't knockback. The knockback is only if you detonate a charge, and then the direction of the knockback is ultra hard to predict (it's related to the charges i think, not really sure. As far as I am concerned, I consider it random).
- for soloing .... There are just better weapon out there.

For FSC, you don't want a Catalyzer at all. Alchemers, Polaris, Needle and AP are all guns worth considering.

I speak from experience.
Neutralizer is very good in FSC. 84 DMG per normal shot (less than Valiance, but who uses normal catalyzer shots all the time anyways?), charge 211. (no bonuses).
It works very well on zombie mobs, eliminating them very quickly. It doesn't work too well on Trojans though. Against vany, I haven't true it on his mask, but it is very good against vany himself. I usually charge, detonate, charge, detonate; makes it do the most DMG I think. It gets harder to use in the later stages though. On the 3rd phase, when he has the 4 orbs around him, fewer charge shots stick, but it isn't too hard to get them on. On the last stage however, I highly recommend putting out the fire with water. It helps tremendously.

I have only met one other person with a neutralizer, and that was about a month or two ago. I'll be uploading a few videos of neutralizer in use in FSC, RJP, and various levels. Can't do it soon though because of midterms ;(

@Fradow Believe me, neutralizer works very well in FSC. 10 charges on one enemy is almost the same as a Levi charge (in terms of KB), and 10 charges will do approximately 2200 DMG on it's victim and a little less than that on nearby enemies. (except for Trojans).

I have both a Neutralizer and a Biohazard, and have brought both of them down to FSC on occasion. Biohazard's not worth bringing there (poor against enemies, poor against boss, even with poison), but Neutralizer DEFINITELY is. It blows through hordes of zombies with ease. As with all catalyzers, the rotation of the charge means that you can sit on the left side of an area and blow charges as soon as they hit, which will push everything away and deal several instances of 200+ damage. I can easily outdamage Polaris users, and even keep pace with Levi users, all while keeping myself safe and sound. I definitely recommend it.

Against Vanaduke himself, it's good against the mask but poor against the body. I usually stick a charge to the mask and fire two shots at it before it gets its shield back up. The rotating orbs block a lot of the shots (due to the slow projectile speed), but as Slayzz pointed out, you can always put them out to make things easier. I personally prefer kiting with Valiance or a Blitz Needle, though, as it's less work-intensive.

@Slayzz, wanna run 2-man with cats sometime? :D

Valiance isn't a pointless gun, actually. The knockback at close range can be something of a lifesaver if you're in a tight spot, and it doesn't have the same distracting Nitronome-like effects as Polaris. It doesn't have specialized damage, admittedly, but that doesn't make it 'pointless'. Don't diss normal-type guns out of principle.

Anyway, back on topic.

Neutralizer explosions are exactly like the Biohazard explosions, ignoring damage type and status infliction. I think it's centred on the point of impact of the bullet with the charged enemy, but I'm not absolutely certain. The gun is awesome for setting up chain reactions, the damage is pretty good, and it's possible to take out a large number of enemies this way (my Virulent Catalyser is particularly a boon when it comes to Lichen Colonies and RJP). With Slag Walkers, I can imagine the Neutralizer would be pretty useful to take out large swathes of enemies if you have the patience for it.

Biohazard isn't much use here. It can take out the Oilers pretty effectively, but the damage reduction in return against zombies and fiends doesn't make sense. It does poison, but to be honest, poison isn't very effective as a status on enemies unless there are mass healers. The poison effect certainly isn't enough to justify the damage loss. I believe that the final stage of Vana is completely immune to shadow - whether you will be able to poison him, I don't know, but you'd be best off with a different weapon.

Out of the two you've mentioned, pick the Neutralizer for FSC runs. (I would personally choose the Biohazard first for other clockworks levels, switching out as necessary since it can do supereffective damage, but since you asked specifically about FSC...)

Neutralizer wouldn't really be my first recommendation for a gun down here though, since it takes some practise to use properly as well as cooperation from your team-mates. Other people are liable to kill all of those zombies with their DA charge attacks before you can detonate the charge attack you used on them. But if you're looking to use it for fun, or for solo runs, and you have the patience to work with it, go for the Neutralizer rather than the Biohazard down there.

Since I have a neutralizer and damage bonus: ultra for guns, I'll test soon when its heated and find out what number I hit at depth 28 ish, charge and normal attack

This gun is by no means bad, it is interesting.