Brachiosaurus

Big Body Little Head. To kill a brachiosaurus by just stabbing it in the legs or stomach would take forever but a well placed shot at the top of the neck where the large muscles holding the neck are small and it is not protected by the bony skull. This can be discerned with a DC 15 perception check or if someone just figures it out. This part of the body has 40 health and 13 AC the Brachiosaurus dies if it is reduced to zero. It is held at 30 ft high at rest and 50 ft high if it is threatened. If this part of the body is inside of an area of effect that deals damage it takes damage but it can not be targeted by other effects.

 Really really big. A brachiosaurus' size allows it to hold up to 10 tonnes comfortably and 20 tonnes while straining. More Than 10 tonnes it cannot make its big stomp Attack

Multiattack. The Brachiosaurus makes a stomp attack and a tail attack

Action Stomp Attack. Stomp attack: +8 to hit, reach 5 ft., 1 target. Hit: 22 (3d10 + 8) [bludgeoning] damage. 

Action Tail Attack. Tail Attack: +8 to hit, reach 15 ft., 1 target. Hit: 28 (4d10 + 8) [Bludgeoning] damage.

Action Big stomp Attack . The Brachiosaurus rears up and stomps with as much of its force it can muster. +4 to hit, reach 5ft, 1 target. Hit Hit: 58(10d10 + 8) [Bludgeoning] damage.