I don't consider the Moon-beast an insurmountable foe even for normal people armed with improvised weapons.
Its physical attributes are high, but still inside the human range (STR 80, CON 65, DEX 50...), it is not specially fast (Move :7), it has only one attack per round, at a mediocre skill (45%) and low dodge (25%), and if you prevent from using a spear, it has low chances of insta-killing a human (it's fighting damage is 1d3+1d4 - at average, it will do 4-5 damage per attack, and its average hit rate will be one [dodgeable] hit every two rounds). It has no armor, and his special resistance to firearms and missile attacks doesn't affects improvised melee weapons. It can be ambushed, it can be tricked into a trap, and it can be overwhelmed by superior numbers. So I don't find it more dangerous than a specially nasty rabid dog. If the PCs are in the "right mood" and try to avoid a direct confrontation, it can be a memorable foe which will frighten and "make sweat" the players, but it is a "final boss" fairly manageable by clever players.
Of course, a Keeper can "go for the throat" and achieve a Total Party Kill loading the Moon-beast with specially nasty spells and playing it as preternaturally aware of the PC's tactics, but I think that such Keeper will have to search replacement players for Episode 2 of the campaign.