Once Human’s Great Ones are the game’s progression spine. Each Monolith boss is a level-gated exam with a loot table behind it, and each one is beaten not by gear score alone but by knowing what to shoot, when it is immune, and what to bring. The pattern across all five is consistent — glowing weak points, an immunity phase that requires an action to break, and an add phase designed to eat your ammo budget. This guide breaks down each Monolith’s fight loop and the counter-play that generalizes across the whole roster.
What it covers: All five Monolith Great Ones (Ravenous Hunter through Mensdevoran) plus the Secret Servitor — weak points, phase behavior, and per-boss counter-play — plus the loadout math that applies to all of them. Why it matters: Every Great One has an “immune until you do X” mechanic that will hard-wall players who don’t know it — the DPS check is the easy half of the fight. Who should pick this: Players hitting their first Monolith wall, and squads prepping Hard/Pro difficulty runs for the mod-tier loot.
The roster at a glance
| Boss | Level | Monolith (region) | Weak points | Wall mechanic |
|---|---|---|---|---|
| Ravenous Hunter | 10 | Monolith of Greed (Dayton Wetlands) | Yellow head sac | Gatling shield + spawner phase |
| Treant | 20 | Gaia Cliff (Broken Delta) | Woman on back, horned head, red vines | Immune until 3 Tangled Vines die |
| Arachsiam | 30 | Mirage (Iron River) | Pus-riddled abdomen; exposed chest/head in mist | Pillar immunity — shoot the spider eggs |
| Rabizex | 40 | Thirst (Chalk Peak) | Head when enraged; shoulders after 2 HP bars | Burrow; orb phase that one-shots |
| Mensdevoran | 60 | Forsaken (Red Sands) | Palms, head, stomach (situational) | Platform destruction + adds |
| Secret Servitor | Rift Space | Onyx Tundra | Three heads | Plasma Beam / Shield Phase |
The general grammar: weak points glow, but most only open after a trigger (enrage, phase transition, immunity break). Fights are cycles — immunity phase → break action → damage window → repeat.
Ravenous Hunter — the tutorial wall
The first Monolith teaches the game’s whole pattern: a Gatling shield phase where frontal damage is wasted, a rocket barrage that punishes static positioning, acid spit for close range, and spawner adds to drain ammo. Counter-play is rhythm: break line-of-sight during the Gatling phase (not out-range it), kill adds fast to keep the arena readable, and dump into the yellow head sac between barrages — interrupt windows that also shorten the add phase.
Treant — the DPS-check cycle
The cleanest example of Once Human’s immunity loop. Treant is invulnerable until you destroy three Tangled Vines around the arena (left vine is cliff-guarded; right vine accessible via updraft cannons — leave one for last so you start the damage phase near Treant’s head). Killing all three opens a 120-second damage window with the woman embedded in its back as the weak point — fire damage is particularly effective, so Molotovs and flamethrowers convert this window into the fight’s entire damage budget. After 120 seconds the vines respawn and the loop restarts.
During the window, Treant still attacks: seismic slams (jump the purple shockwave ring), homing white orbs (shoot them down), and the bound woman’s five-second laser (hard cover — the broken bus — blocks it fully). Cyst Seeds spawn on the ground; breaking them gives protective domes. On Pro difficulty the fight becomes a hard DPS check — fail to break enough health in the window and the scream attack will end you.
Arachsiam — crowd control homework
The spider’s kit is a web gauntlet: a web-pull that drags you to her (roll out the moment it releases), web projectiles that entangle on landing zones, a visibility-killing mist phase, egg-laying that spawns spiderlings, plus well-telegraphed jump and rush attacks. Two rules break this fight:
- When she climbs the pillar and goes immune, shoot the eggs at the web-bridge ends — that is the only way to pull her back down. While perched she keeps webbing you, so dodge on sound cues.
- In the mist phase, her exposed human head and chest become the new weak points — high-burst weapons (shotguns, bows) convert the visibility problem into a damage window.
Bring volume: guides recommend ~2,000 rifle rounds plus sidearm ammo and 20 activators for this one. Assault rifles answer most of her patterns cleanly.
Rabizex — respect the orb phase
Rabizex opens with no exposed weak point at all. The sequence that matters: it burrows, emerges enraged with the head glowing (your target), and after two HP bars drop, the shoulders open between AoE pulls. The kill-or-die mechanic is the orb phase — a one-shot wipe if the orbs are not handled; accuracy there is the entire fight. Shadow projectiles and AoE pulls punish greedy positioning — treat it as a mid-fight DPS race with a bullet-hell mini-game attached.
Mensdevoran — the endgame set piece
The level-60 exam mixes everything: slam and sweep attacks that destroy the platforms you are standing on, add waves, a grab, and a laser. Weak points rotate with the fight (glowing palms, head, and stomach situationally). The counter-play is arena management — keep two escape routes at all times because the floor is a consumable — plus the standard add-clear discipline to keep the pressure readable.
The universal loadout math
Three pieces of preparation generalize across every Great One:
- Living Armor potions — crafted from the Harveseed deviant (a Treant drop), this grants a one-time death shield (once per 10 minutes, ~1.5-hour duration). For one-shot mechanics, it is the single best consumable in the game — and it makes your first Treant kill fund every later fight.
- Master Tactician + throwables — the skill grants +25% boss damage for 15 seconds, triggered by throwing daggers or snowballs (Snow Globe deviant spams the latter). Free, repeatable, and stackable with weak-point hits.
- Recurve Crossbow + Bullseye mod — weak-spot hits inflict Bullseye (+30% crit rate), with a Vulnerability mod slot adding damage on top; found in a mystical crate at the Bell Tower near the Evergreen Rift anchor. For armor: Renegade (damage stacks to 40%) or Falcon (crit-focused).
And the general rule the whole roster teaches: ranged pressure wins. Every Great One punishes proximity — the recurring meta is weak-point sniping from range with movement between phases, not face-tank DPS.
Next read
- For the territory half of Once Human’s loop, see Once Human base building & anomaly defense.
- For where these fights sit in the genre’s PvEvP spectrum, see Survival-craft in 2026: the core loop explained.
- For the game’s console launch and October Ocean Expansion, see Once Human lands on Xbox Game Pass.
SOURCES
- Theria Games — Once Human Great Ones: https://theriagames.com/guide/once-human-great-ones/ — primary roster table (levels, regions, weak points, mechanics), loadout math (Living Armor, Master Tactician +25%, Recurve Crossbow Bullseye, Renegade/Falcon sets), and phase framework.
- Theria Games — Once Human Treant: https://theriagames.com/guide/once-human-treant/ — the 3-vine/120-second damage cycle, Cyst Seed domes, fire weakness, vine access routes, and Pro-mode DPS check.
- Game8 — Arachsiam Boss Guide: https://game8.co/games/Once-Human/archives/Bosses-Arachsiam — egg-shooting immunity break, web attack catalog, mist phase weak points, and supply budgets.
- Gamepur — How to Beat Treant Boss: https://www.gamepur.com/guides/beat-treant-boss-once-human — laser cover behavior, shockwave dodging, platform positioning, and the level-15 Normal framing.
- Xbox Wire — Once Human on Game Pass: https://news.xbox.com/en-us/2026/09/07/once-human-on-game-pass/ — platform context (free-to-play, crossplay/cross-progression).