I'm thinking a LARGE (more like MASSIVE) group of Morelul guarding it, several Bulbasaur and bulky Ivysaur guards, MANY Paras and some Parasect who have the move sleep powder to put the team asleep, and several other grass type poison type and bug types. Shiinotic himself with his large mushroom head can be very defensive and as a result could be much larger than most Shinnotic, providing a great bulk for the boss pokemon. In addition, the woods will serve as a puzzle, a maze. The team wont know the herb is a mushroom, so will go around picking wild leaves. This will end up with wild pokemon attacking them such as Oddish, Sewaddle, Tangela and Leafeon. There will be a field of sleepiness in which entering pokemon gradually get sleepier and eventually fall asleep, the energies of which are guarded by psychic types like Munna and Drowzee.