Arr... if memory serves me be, the Druid can attain physical immunity in Water Elemental form, except for Crushing damage, which will be around 50%+.

That is alright though, since Shambling Mounds with the War Chant of Sith have immunity to Crushing Damage.

So your Druid tanks the Slashers and the Piercers, while your Mounds tank the Crushers.