I love the biohazard. but here is a tip you might not even know.

If you get two enemies close together you can place one charge on each with a single charge shot if you place the bullet right (can't do with AutoTarget on.)

I know that when i 1st time use toxic after freshly craft it love it even more

I've owned a Biohazard (and eventually, Neutralizer) since they came out.

Facts you should know while using them:
- Charges last for 20 seconds before they disappear.
- Charges always appear on the left of a monster (so in a group, if you're doing a single charge + instant explosion, you always want to aim at the right-most enemy, not relative to the direction you're facing). So if you're using a vortex and you're above (north) of it, you want to aim at the top/top-right enemy (because they move) then try to detonate it as it moves towards the Northeast/Eastern side of the cluster.
- If you're using charges, CTR will boost your "fire rate" the most, but ASI is also significant, because it lets you start your next shot earlier.
- You can't land a charge if a monster is invincible (or if you hit an enemy's shield), but you can still detonate any pre-existing charges.

Other tips:
- If you have the aggro of a single monster, you can load it up with charges (much easier to aim when they're moving towards you), then push it into a crowd and detonate. I've done this with one of the older versions of the Royal Jelly - jammed as many charges possible on a single jelly while the Royal Jelly was invincible, shield pushed it into the boss and detonated it. If you have a hail driver to freeze with, this is even easier.
- If not far away and being chased, shoot charges as counterattacks to your enemy's melee attacks. Especially useful with the jelly enemies, but works on Kats as well, if you know how/where they will move.

Lockdown
- CHARGE IS BROKEN. I just did a recent test with VH damage biohazard: 4.5 bars of damage with normal shot, 1-1.5 damage with charge on the owning target. This isn't even a case of invincibility frames - it's broken.
- Use regular shots to block off your enemies' movements. The easiest to block are auto-target swordsmen - they tend to move directly towards you, so you can drop some timed shots to keep them from using anything more than their first slash. Versus more experienced people, you can use it as a slow wall to interrupt a striker movement while you switch to another weapon to fight off their other approaches. Again, baiting the actual attack is important, so it's actually useful as a mobile close-range weapon. I will often tell people I "punch" with Biohazard.
- Blocking movements, 2 shots at a time: A common move done by valiance owners is to shoot in a "V" pattern extending out from your character. This is also useful with the catalyzer lines because it can either mentally or physically block your opponent from approaching in those directions. The closer they are, the wider you want your spread to be.

I love my Catalyser, it's a wonderful anti-Gremlin tool IMO. I find triggering their dodges to be very useful for keeping them from dog-piling me, so I just spam the normal attack and it does plenty enough DPS. Against tanky monsters (like the Lumbers that I often see with Gremlins) and against big gobs of Slimes, (or Scuttles, which also occur with Lumbers and Gremlins...) the charges offer great DPS from a safe distance.

I did not know about the damage being blockable by the charge being inside a nearby blocking object, thanks for pointing that out!

I agree re: combining with Magnus/Spinecone knockdown and I suggest also trying a Cat with any freezing weapon. Attaching cat bombs does not undo freeze, so you can stack two or three with relative ease. I'd be doing it myself if I weren't so much more fond of Fire... I find that my Dark Thorn Blade's third hit (often easier than hitting with a full combo, and their shield bubbles make first-swinging them to death too slow) or Blazebrand's charge attack reliably knocks Menders down, and one or two Virucat shots is enough to stop their healing shenanigans.

I also find my Virucat especially valuable for its normal attacks. IMO the Blaster line and all of the Roarmulus guns are VERY useful for the ability to attack with very little movement disruption and be able to use a shield very quickly afterward if desired. I seldom go anywhere in the clockworks without at least one of those 3 types equipped, because that's what I fall back on when I don't have enough room to handle monsters in easily manageable bites. (YMMV: I imagine the Jelly guns are valued for similar defensive mobility, but I find myself derping into a forced reload by losing track of how many shots I fired and, to me at least, it feels like Antigua gives me less DPS in proportion to how much it slows me down even when I do avoid reloading. At the other extreme, Alchemers also feel like they slow me down too much, so I get a "Goldilocks" effect while some others seem to prefer one extreme or the other for most safely mixing it up. For that matter, I get the Goldilocks effect with swords in terms of both defensive and offensive properties, so maybe combo size preference in either the gun or sword class gives a good idea of where preference in the other class will likely be...)