As far as bang for your buck mercurial isn't really the way to go, as I understand it the speed increase is very minor and only really helpful to very rich bombers who can already reach max ctr and damage with UVs and trinkets. If that's not you sticking to the standard bomber armors (ones you don't have to do shadow lairs to get) is probably best especially since volcanic demo with its fire resist is very useful in FSC, the most profitable levels in the game.

Alternatively the gunner lines are ok too or you could go with something like a skolver coat/divine veil (maybe subbing in justifier jacket instead of skolver for the gun buffs) combo which is not specialized but does cover all damage types and some of the worst statuses.

As far as weapons go I second what nordlead has to say, specialization is usually the way to go and really only requires one or two extra weapons to switch in and out. For guns the alchemers are all good for elemental and if you get voltech or firo the status effects work great on jellys too, magnus is good for pierce and good at knocking hitting dodging gremlins (see what I did there? now you don't need a shadow gun).

As for bombs status bombs are usually more helpful to parties than nitro. Status bombs are some of the most useful weapons in arenas, royal jelly palace (rjp) and firestorm citadel (fsc), the three places where you make the most crowns. The Ash of Agni line (fire) and Voltaic Tempest line (shock) stack really well together and when all of your enemies are on fire and chain shocking each other they die real quick. The shivermist buster (freeze) line is great for crowd control and for use in the final boss fight.

And once you can afford 4star stuff you should invest in a third weapon slot, by that point 250ce a month is not that much (or even just 150 ce if you save your mist) and it reeeeally helps you be more versatile.

For shields if you end up grinding rjp a lot make sure to snag a brute jelly shield. It has better pierce resist (which is great for further rjp grinding) than the snarby shield line and since you're doing bombs/guns you don't need the sword buff from the snarby line. And since you want to be efficient it's basically a 3star shield for 3500cr (the opportunity cost of not selling it to vendors) which is super cheap for 3star gear. When I was working on my pure bomber alt and trying to be super stingy with my crowns I got a bristling buckler from snarby, used it to grind rjp till I got the brute jelly then vendored the buckler.

But yeah Grey owlite should also be a goal for you in terms of shields, the elemental defense is really useful, it's definitely the shield I use the most by far on my main. And remember, you can't use your shield while charging bombs so while you'll still need it for bumps or if you get trapped the more you end up bombing the less you need your shield.

And if you haven't read the wiki and guides do so, they can be quite helpful.

Pick up a shadowtech. Attack patterns are the same as a cryotech. It also covers your damage bases, dealing effective damage to 4 families, and neutral to the other two.

Personally, I prefer other alchemers over the cryo. But that is preference. What's known as fact is that cryo has the lowest dps out of all the elemental alchemers. The usage of the freeze may or may not appeal to you.

There is a thread titled "experience with hail driver" somewhere in the arsenals. Perhaps on the 2nd page. Or 3rd.

If, after your own consideration, you wish to obtain a different elemental alchemer, I suggest nova for pure damage. If you wish to have a status, voltech, which applies shock.

Don't sell weapons, you might have a desire to use them later.

Have you checked out the damage page on the wiki? Go read that. Then, go read the page on monster, or perhaps it was called family.

There are 6 familes, each with a different weakness and a different resisted damage. There are 3 damage types, piercing, elemental, and shadow. By having 2 of these damages, you deal the effective damage type against 4 families, and neutral damage, which is base, to the remaining 2.

Nova and voltech are both elemental, so you'll deal effective against elemental weak, neutral against elemental neutral, and ineffective damage against everything that resists elemental.

My suggestion was a shadowtech, dealing shadow, to go along with your cryo. Then I said you can replace the cryo with either nova, or volt.