Arcanamech

Pilot controlled. The Mech itself is not a being. there for it takes on the Wisdom, Intelligence, and charisma of the pilot. if the Mech is destroyed the pilot will still be alive at 1 hp. if the pilot gets out of the Mech the pilot will return to its max hit points or the current hit points of the Mech, whichever is lower  

Immutable Form. The Mech is immune to any spell or effect that would alter its form.

Magic Weapons. The Mech's weapon attacks are magical.

Mendable. If the mending spell is cast on the Arcanamech, it regains 1 hp. However if the caster spends an action and has smiths tools they can heal the mech for 2d10, this does not work if the mech is destroyed. 

Siege Monster. The Mech deals double damage to objects and structures.

Magic Resistance. The mech has advantage on saving throws against spells and other magical effects.

Cockpit. The Mech has a large cockpit in the center of it capable of fitting a creature of large size or smaller. Once inside the cockpit, a magical field protects the pilot from magical attacks and provides it with half cover from any outside physical attacks. 

Multiattack. The Mech makes two melee attacks. One sword attack and one slam attack, or two Magic Missile attacks. 

Slam. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Sawblade. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 6) slashing damage.

Magic Missile. Ranged Weapon Attack: +6 to hit, range 60/120 ft., one target. Hit: 15 (1d10 + 5)

Flamethrower (Recharge 5-6). The mech's right arm changes into a powerful flamethrower and shoots out a large, 30 foot cone of blue fire that engulfs everything in it's path. Anyone within the cone must make a DC 15 dex saving throw or be burned taking 26 (3d10 +5) fire damage. 

Thunder Saw (Recharge 5-6). The mech's saw blade cracks with electricity. Digging it into the ground and rupturing it. The ground cracking and rumbling underneath the mech and any creature within 10 feet of the mech must make a DC 16 dex saving throw or be knocked prone as well as taking 7 (1d8 + 3) lightning damage. The area becomes difficult terrain and anyone who starts their turn in it takes 1d4 lightning damage.