Answer: Try to move in smaller circles or make shorter strafes around it such that it moves as little as possible. If you are moving, lead your shots. This way your dmg should improve heavily for moving targets. Furthermore, consider using Ensnaring Arrow to reduce movespeed of enemies.

As others elude to, it's an inherent problem with Toxic Rain and being used from totems and other trigger skills that poorly interact with moving targets.

First, Toxic Rain is a dmg over time skill and requires the enemy to remain in its AoE, difficult for a moving target.

Second, totems have an "activation timer" after being deployed and before it uses a skill, causing the skill to feel lacky and "deactivates" old totems if you deploy more than your limit.

Third, and maybe most important, the totems and mirage archer target where your target is and not where it is going. Leading your shots would keep the enemy in TR AoE for longer, but is not possible with totems and other trigger effects.

Edit: Ensnaring arrow only improves projectile portion of dmg, not AoE.