Gunner has been very strong for a long time, probably since the mid-to-end point of EP4 but flew under the radar just because a lot of people don't play the class. It's the only class that has an ability that can actually scale with party contribution as Chain Trigger can be built by your party members, meaning if you're running with at least 2-3 other people, Chain 100 is possible for most of your Chain targets.

Until recently, Gu mobbing was entirely based around Reverse Tap and Shift Period which worked but really weren't great tools for it. Now we have S. Roll Arts which is a cookie-cutter tool for mobbing and basically everything else which is why suddenly it's in everyone's sights now despite a ship of 2 Gu, 1 Ra and a Te being a standard MPA Boss Killing Squad.

Here's my stats:

https://imgur.com/a/w1z0S2o

This is just what I did - there may be a more optimal way to spread out stats but this is what I've found works for me.

Atra TMG is S1: Double-Edged Sword 4%/S2: Crit 4%/S3: Crit 4%/S4: Life Steal/Astral Soul/Phrase Weak/Grace Shoot. During Boost Week I'll probably re-affix to 8s and slap Shoot 6 on there.

Izane Arm is Ares the Soul/Shoot 4/Stamina 4/Arks Max/Omega Reverie/Grace Shoot. My Buster Leg isn't pictured here but it has the same affixes as my Izane Arm.

Weila Rear is Til the Soul/Shoot 4/Flict Tiro/Arks Max/Apprentice Reverie/Grace Shoot. I used to rock the same affixes I had on my Izane Arm/Buster Leg but realized I had too much HP and could go for more R-ATK, so I re-affixed for that.

There is enough HP and defenses there to so that little mistakes that I make usually don't amount to a death. I mentioned a few pages back but I did happen to eat a Dragon RM fireball by accident and it only dealt me a little over 1000 with Fire resist drink, so something in that HP/defense range may be what you're looking for. I'm even thinking about potentially dropping some more HP to get more R-ATK out of my units.

Small note too - you'll want to keep Quelle TMG on your bar even though a well-made Atra is better than it because of the 5 seconds of invul you get on activating Chain Trigger with Quelle TMG. It's good to have as a crutch if things go wrong (e.g. didn't kill enough cubes in Mother cube phase) or if you want to set up Chain during a series of attacks (e.g. Deus arm slam or Hunas thunder bolt phase).