The last Raid Level 3 boss Achates cannot be soloed without abusing travel animations (tight rope or ladder) to avoid the curse debuff. The curse debuff requires 2 people to properly do the mechanic, so it should not be present in a solo instance.
In addition, the statue mechanic goes by way too fast. I was picking up and throwing the stone as fast as humanly possible (my sharpshooter eagle destroys the statues the second they spawn) and I still didn’t have enough time to break the shield. The colored fragments shouldn’t despawn until the shield phase is over and the phase itself should last a few seconds longer.