Colossus Engine

Burst Motion. The Engine only acts on every other turn. On it off turn, it has an AC of 16 and a Dexterity of 7.

Steam Engine. When the Engine is slain, it explodes in a blast of hot steam with in a 10ft radius. Any creature caught in it must succeed a DC13 Constitution saving throw or take 2d10 Fire damage, or half as much on a success. 

Magic Catalyst. Once per turn (both on it on and off turns), the Engine can reduce damage from a spell by 1d4 + 1.

Multiattack. The Engine can make 2 attacks with its action.

Slam. Melee Weapon Attack: +7 to hit, reach 15 ft., 1 target. Hit: 13 (2d8 + 4) Bludgeoning damage. Miss: deals 6 (1d6 + 2) Thunder damage to any creatures within 5ft of the target creature. Has disadvantage on creatures within 5ft.

Stomp. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 11 (3d4 + 4) Bludgeoning damage. Hit or Miss: all creatures within 5ft of the Engine must succeed a DC 14 Dexterity saving throw or take 6 (1d6 + 2) Thunder damage and be knocked back 5ft.