Zardnaar
Legend
How is that even possible? Even if your party is weak at range (i.e. not built to take advantage of a great tank), a Gargantuan creature doesn't block movement of Medium or Large creatures. Other PCs can get past you if they want to just by paying the extra movement cost for difficult terrain.
Of course not--aside from the newness of Volo's Guide, pure shapeshifting isn't even a good use of the Moon Druid's skills, so I'd expect never to see one of those in play unless a player is enjoying goofing around. (And in that case it's probably more likely that I'd see a Shadow Monk/Moon Druid combination first, who teleports above an enemy and then turns into an earth elemental or triceratops in midair to squish the enemy. A player has already tried that one, but the PCs didn't make it high enough for his strategy to come to full fruition. Also, he got distracted by events in play and wound up emphasizing Shadow Monk advancement more than Moon Druid, despite his original plan.)
Despite running what may be an unusually-lethal campaign with high PC turnover, I've only seen maybe 1% of the possible builds/strategies in play yet. There are thousands and thousands of permutations; hundreds of them are even good permutations.
Your party members count as difficult terrain.