You have to clean all the rooms. Once the pulse is destroyed, you must empty then fill every room to have clear water and makes trashes disappear. Then the path north to Titania will open
The field will be changed for the misty terrain for 5 turns (the duration of mist) then returns to mirror field or whatever the leader's field is. So if you can't use a field at your advantage, you can just remove it
And Steel is super effective against Ice. But I assume Serra has something to counter such a weakness
Or Mamoswine with mist. Or Magneton/Magnezone with electrik terrain. It's always useful when you can't break a field
And in the other way, special attacks never miss. So while Serra spend turn bossting her evasion, you can still hit her