Arcane Ballista

Mind of the Master: This construct has the same Intelligence Score as whoever is controlling it , unless said  Ability Score is lower than the Arcane Ballistas inital number. It can be commanded via Bonus action by its creator or whoever else currently commands it and will do what it is told to its best abilities.

Axiomatic Mind. The Arcane Ballista can't be compelled to act in a manner contrary to its nature or its instructions.

Spellcasting. The Arcane Ballista is a Cantrip-level spellcaster. Its spellcasting ability is Intelligence (spell save normally DC 14, +6 to hit with spell attacks).  The Arcane Ballista has following Artificer spells prepared:
Cantrips (at will): Magic Stone, Mending. 

Selfdestruct: Once the hitpoints of a Arcane Ballista reach zero,it detonates,causing it to explode into shrapnell and fire. All creatures in a radius of 10ft must succeed a Dexterity saving throw (DC 14) or suffer 2d10 fire damage . Half as much on a success.

Rapid fire. Once  Magic stone(range 60ft, 1d6 bludgeoning+Int) is active the Arcane Ballista can use  up to all 3 pebbles as a multiattack action.(3/1 day)

Self Repairs. The Arcane Ballista can use its action to cast mending at Self range , healing it for 2d8.

Burrow: The Arcane Ballista can choose to reduce its speed to zero at the start of its turn as a bonus action by jamming its spider like legs into whatever surface its currently on. During this it has advantage for Saving throws or Checks against being dragged , pushed, shoved, or knocked prone.