Flag your team so they don't attack the guardians. If you are running with heroes disable attack skills and let minions die before running through that area.
Drop the 'all' flag, drop your individual hero flags, then dismiss the Hero flags, and your heroes will follow you and the henchmen (if any) will stay put (and if you have a MM hero, then leave them behind too). Then set your heroes to avoid combat, and switch to guard if you need help with the moss scarabs only.
Just flag your heros/hench before you start bonus and flag dunkoro to come with you (guard)...then once over the log unflag dunkoro so all you do is when the scarabs attack you set him to fight also if needed then set him back to guard for next seed drop