So I've been trying out all the swords and I have definitely taken a liking to the flourish series. I use a gun as my backup weapon and Im having trouble deciding which one to get. any ideas?
What's a good gun to complement the flourish line
If you'll only have the flourish and the gun, I'd suggest the blaster series to start out because it's good at taking out gun puppies. I now use a calibur, flourish, freeze bomb, and voltech driver and at 4* the range finally seems long enough to safely take out gun puppies.
Maybe cryo driver would be good too. It's very versatile and I think it would work well with your flourish. The other alchemers would be fun too if you have a backup plan to oilers/quicksilvers.
I would say don't limit yourself. If you're not in a huge hurry to get into 5* gear, spend some time with the different 2* guns. You can get them all from the AH for a decent price or make them yourself (the recipes aren't too tough to find). You may find that you want a voltech alchemer for construct levels but a shadowtech for jelly strata. The good news is you can have them all, and reequip at the terminal between strata.
Cryo is my favorite of the alchemers because it freezes puppies so you can get behind them safely, so that would be the first gun I'd go for, but it wouldn't be the last.
Just a quick note about the Alchemer line of guns: There is a VAST difference between the 3* and 4* versions of the guns. The bullets of the 3* have a small chance to ricochet, but generally don't. The 4*'s bullets pretty much always do, in addition to being faster and considerably more powerful. The 4* Drivers will consistently do about twice the damage of their 3* parents, and on groups of enemies will net you a considerable amount of damage if you're careful or get lucky with the ricochets. (Note that the 4* gun's charge attack can split TWICE!)
I'd recommend the Cryo Alchemer line, largely because you can't accidentally set fire or shock stuff you don't want to with it. The Freeze is useful for keeping stuff in place for other gunners and/or bombers to hurt, or just generally out of other people's hair.
There is something to be said, however, for always carrying a normal damage-type weapon at all times. Especially if you only have the default 2 weapon slots. If you want to keep the Flourish, the Blaster line isn't a bad fallback option.
More than likely, if you're set on getting a gun for your second weapon, get an alchemer, preferably the cryo, volt, or firo versions. Those are all elemental, so will hurt the constructs your flourish will fail at damaging, and do OK damage to slimes, and all provide some status effect to your enemies. Just watch out, and don't shock Quicksilvers or set fire to Oilers!
You could also go for the blaster series, but that's boring ;) Really, it's all preference, but elemental damage is probably what you'll need more than shadow, and your sword does plenty of piercing.