"Death from above" Gemling Legionnaire Artillery Ballista Build

https://maxroll.gg/poe2/passive-tree/av25k0q1

Swap strength nodes for dexterity as necessary to meet gear requirements.

For the ascendency, I went Crystalline Potential first to boost my damage in a non-ballista build, but Adaptive Capability may allow this build to come online earlier.

The tree here is incomplete. I need to feel out where to go from here. The last totem cluster is an obvious direction to go, and the Total Incineration cluster is that direction, too. Alternatively, you could spec into Rage and put Font of Rage on your totems, or build more defensively with things like the Prism Guard cluster, the Leather Bound Gauntlets and General's Bindings cluster, Backup Plan, etc. Depends how it feels in maps.

The last two ascendency points are either Advanced Thaumaturgy for the reduced costs, or Enhanced Effectiveness for the extra HP. Again, it depends what I feel like I need.
https://maxroll.gg/poe2/passive-tree/ze2630q1

Crossbow with +# to Level of Projectile Skills
Whatever decent gear you find

Bind your Artillery Ballista to the +Level crossbow, bind everything else to the best crossbow you've found so far.

Flat added damage on jewelry and gloves does add to the ballista damage, so it can be a nice bonus, but I'm clearing fine without it. Movespeed on boots is always good but, again, I don't have any and it hasn't ruined anything.

Primary skill:
Artillery Ballista with Martial Tempo, Fire Infusion, and Primal Armament

Overabundance might be more damage, but I don't like having to resummon that often. The last two supports will probably be Immolate and Font of Rage, but Elemental Focus, Fire Mastery, and Fire Penetration are also potential options.

Support skills:
Flame Wall with whatever links you prefer

Keep this at level 2. Its just to ignite enemies since a lot of the fire clusters are "increased damage to ignited."

Glacial Bolt again with your preferred links

This serves three purposes: 1) it helps contain enemies so your ballistae can hit a bit more reliably, 2) it occasionally freezes enemies, 3) it can block projectiles. The big orb in the skelephant fight? Blocked. The lightning in the titan fight? Blocked. It won't block everything, but I've appreciated every time it meant I didn't have to dodge frantically. Do note, if you run Fortress in this, that it can start spawning the ice wall behind you and block off your escape.

Flammability linked to Burning Inscription

For bosses and tanky rares. Burning Inscription isn't necessary, but it makes sure the boss is burning when their fire res is debuffed for all the damage.

Auras:
Overwhelming Presence
Time of Need
Attrition
Vitality
Clarity

Pick as many as you have spirit for. Time of Need and Vitality help counter the health costs we pick up from Price of Freedom and Blood Rush. Clarity, similarly, helps with the high mana costs from our level-pumped skills. Overwhelming Presence makes stuns and freezes a little easier. And Attrition helps with bosses, since we're not just destroying things.

Extras:
Incendiary Shot with Fire Exposure
If the damage gets high enough to reliably ignite, this could swap in for Flame Wall. It should be better damage overall, but I'm using a bunch of the good fire supports for the ballista, so it's tough to get this to hit hard enough. Also, then you have to swap ammo.

Siege Cascade
It combos nicely with Glacial Bolt, since it considers all the ice wall sections to be enemies, but we don't really get the full power from it since we're not freezing often, and not pinning at all.

This is probably obvious to anyone who's run a totem build, but put the ballistae in front of you, try to block enemies in with the ice walls, and use fire wall to keep them ignited.

I haven't tried any of these, so take them with a grain of salt, but I'm pretty sure this build will also work on Witchhunter or Warbringer. I'd expect a Warbringer version to be a bit tankier, since they can share damage to the totems, and a Witchhunter should be better DPS, with some combination of Culling/Decimating Strike, Concentration, and explosions.