Actually the summons are doing just about as much damage as you'd expect for a slightly above beginner level skill. (I mean, Lilim's lightning bolt attack does about 100 damage).
And yea, she would deal more damage if she was faster as she could fire more shots (Six at a time will be the limit though for each use of Mana Revolver since Revolvers only hold six shots) and if her INT was higher. You can focus on learning new abilities (in fact I recommend it, like learning new ways to power up your mana shots or to fire more powerful shots with your revolver).
I mean, 180 Damage against a Level 23 monster is actually really, really good (especially with its Resistance, which is actually pretty good), especially for a skill you got at low level. Of course, remember that higher level opponents or more skilled opponents have ways to block stuff like this.
You aren't falling behind yet, but I guess after this all of the characters will be considered falling behind (not just Sasha, even Ren whose Level 20 will be considered falling behind).
Remember, you guys have only had the power for about a week now. That's not really a whole lot of time to learn anything useful. But yea, right now for a Level 15 Character, Sasha has a pretty powerful skill under her belt that grows stronger the more DEX and INT she has (plus increasing her Focus Score will help augment that as well). So actually she could indeed keep leveling Mana Shot to increase its base power and then the breathing technique more to power that up more
(which you're right the Breathing Technique was up, ill edit that later, gotta study for now)
So yea. Mana Revolver can be your bread and butter, I just recommend a more powerful Mana Shot. Sasha isn't behind on any means. She could even use her Enchanting to eventually make some sort of ring or item that goes on her hand to power it up even more (something like Yuusuke's Spirit amplification ring from the very beginning of Yu Yu Hakusho, only this would work for Mana).
And do remember that I made this system up on a whim. If there's ever going to be a reboot/sequel to this, then I will flesh out those details a bit more, but right now even I am still balancing things and changing things XD