Mercury's Trial on expert shouldn't pose too many problems if you know what to look for in it. Bog isn't the worst status ailment. The worst outcome though is getting hit point blank or trapped in wall, followed by an aqua spiral while bogged. Other than that, stay off his back and stay moving.
Tip for all Dragon trials:
Don't stand directly behind or you trigger a tail whip that deals major damage
Notes for Mercury:
- !! forward close range aoe head smash
- non telegraph, leans head back and fires a projectile forward. Easy to dodge as it is slow moving
- Aqua Spiral is a giant point blank AOE from his center that generates a whirlwind around Mercury that deals heavy damage + bog. Varies in size. Believe it is based off Overdrive in effect. Smaller versions are inside range for range dps. Residual can leave floating water bubbles
- Summon Help calls 4 mobs forward that do cone water attacks inflicting bog