upgrade the avenger if you want a divine avenger, the 5* is completely better, you can also upgrade the vaporizer to 5* because it is more useful the bigger its radius

as for krogmo, both are good, electron if you use it as its radius will increase, if you dont use it at all and use the needle then the extra damage will help some

In case you tl;dr Electron is better for general dungeoneering. Needle is better for fighting vanaduke.

Okay, in general use, I believe the vortex bomb will be more useful than the toxic needle. In personal opinion, toxic needle is inferior to blitz needle, because what I look for in that gun is raw damage and blitz has more of that, that only means I haven't found the perfect use for the other one, and while it would be inferior it doesn't mean it outright sucks as it works the same.

However, electron clashes with shivermist, they work different but do the same, disable enemies. And again in personal opinion shivermist is better at that. Shivermist has weakeness though, for instance it doesn't work at all on ice-type enemies, so if you like the role of disabler, the vortex will be useful there at the very least, and it may be that you like it more than shivermist and use it everywhere else.

While that is general use, I would aim for a FSC set. On a FSC run there are several roles a player can take, given the weapons you have I would go with:
Vog Cub
Owlite
Needle
two of Calibur line/Avenger line/Pulsar, the last two offer more possibilities, but no mix is wrong
Shivermist

Because that set covers every single role you could have. Which means given your party and looking at what they do you can switch your weapon and do what your party is missing. Of course you may limit yourself to less than four weapon slots so nevermind then. Also while I include a needle line weapon if you go with pulsar you will be able to cover its role, pulsar isn't that great at it, but will do its job, so if I were to have one slot less I would cut the needle out giving more importance to the electron vortex perhaps.

Now about synergies, needle will work great with shivermist, as you will have a defenseless target to shoot at point blank range if you wish. Piercing damage is great versus beast and fiend, but guns are not advised versus beasts, so that leaves it's use to nuke devilites, greavers, and trojans. Out of them trojans will be the best target, for greavers and devilites you will probably have to shoot as they spawn on an arena or better yet freeze them first. In general play your other weapons, while not having extra damage, may end up safer. your sword charges compliment perfectly with the immobilizing effect of freeze and they do normal damage (and arent guns) meaning they have better synergies. Needle will excel if someone disables the enemies for you which means you have all the time in the world to make the perfect shot (which is why its included in the FSC set, if someone disables you can use needle, if no one is, you switch to shivermist, or electron should you like it more).

I don't think electron works great with your weapons, perhaps with shivermist, perhaps with pulsar. You will probably end up confining you to the disabler role anyway and let your teammates do anything else, assuming they can work their way around your bomb. Don't worry that's needed even with shivermist, some people decide its a good idea to attack the enemies face to face, or even charge an attack standing right in front of them, many times you will decide its safer for your team if you stop helping them with your bomb since they will start dodging again :P