You can absolutely win.. however.. you need to 1 exploit NPC tactics as much as humanly possible and 2 get very lucky on what mcslashyoneshotface decides he wants to do. Your best bet once Dune arrives is to teleport mccheesyfuck on the exact opposite side of the worm as your party. Hoping his AI decides the worm is his target and at the same time the worm decides to hit him instead of focusing your caster every gdamn round
I do not think this can be done legitimately at the current softcap of 7 (Covered most the map not close to 8) by legitimately I mean walk into the encounter room and face it as intended.
Even trying to place ranged on the wings is sketchy because Larian thought this encounter wasn't cheesy enough so they placed oil barrels on the wings and an NPC AI programmed to ignore all tanks wide out in the open and phoenix diving to that spot on round 1 before the ranged wits oriented rogue acts.
The DPS of every NPC other than Sandworm (Hes cool in my book) needs to be reduced and the absolute hardon Larian has for all NPCs acting first regardless of role needs to go out the window. Lengthen the fight with hps or something but make it less reliant on cheese/cc spam/do not get hit ever. Let the player position his party or at least spread out I swear I am getting Baccus Rex flashbacks with the round 1 first initiative AoE party wipe fire rain.