Ok, guns are awesome for solo, because you can take as long as you want to and kite safely to avoid losing health.
Because you can do this, the damage output of guns is considerably lower than that of swords.
In a party, this heavily penalizes you because:
1) You are far away when health pills and other usable items drop, so don't get anything unless nobody else wants it
2) The sword and bomb users are knocking targets back and forth, making it impossible to hit reliably
3) The sword users are taking hits like mad, and chugging down health pills like they are in unlimited supply. Your ability to kite and avoid damage is meaningless since the only time you NEED to is when you are all so overwhelmed that the sword and bomb users are destroyed and it is impossible to kite
To add to the issue, gun charge attacks are HORRIBLE. They do almost no additional damage as compared with the basic attack from the gun. A few guns have "special" charge attacks that fragment on impact. On occasion those fragments actually hit something, but typically they don't do much. I don't know of anyone who actually charges their gun when solo, but in party they do because then they LOOK like they are doing something at least.
I propose a small change to the charge attacks to make them more useful in groups. Split the guns into 2 groups, possibly with the same division now in place for guns that can fire while moving and guns that make you stand still to fire.
Snipers: These guns make you stand still when charging their power attack. You cannot move until you fire the shot.
Quickdraws: These guns allow you to move while charging them, but instead of hitting max charge and holding there, your charge will raise and then lower rhythmically. There is a medium sized period where the shot will be normal, with a small period where no shot happens, and a small period where a powerful shot happens.
With these changes, the use of your charge attack requires either planning (Sniper) or skill (Quickdraw). As a reward for the extra effort, the damage will typically be far higher than that of a sword or bomb attack. In most cases, a single target attack is the end result, but some "Beam Cannon" guns could be developed which attack all enemies in a line when fully charged.