Clockwork Guardian

Overclock (Recharge 6). As an action, the clockwork guardian overclocks itself for 3 rounds. While overclocked it increases all of its attributes by 4 and spends its bonus action each round making an additional attack or using the Dodge action. While overclocked the clockwork guardian gains advantage on attack rolls, a +3 bonus to damage, and resistance to bludgeoning, piercing, and slashing damage from nonmagical weapons.
           Roll 1d20 whenever this feature is activated. On a roll of 1, the clockwork Guardian goes berserk in addition to their normal bonuses. On each of its turns while berserk, the clockwork guardian attacks the nearest creature it can see. If no creature is near enough to move to and attack, the Clockwork Guardian attacks an object, with a preference for an object smaller than itself.

Multiattack. The Clockwork Guardian makes four katana attacks and mounted crossbow attacks in any combination.

Katana. melee weapon attack: +7 to hit, reach 5 ft., 1 target. Hit: 10 (1d12 + 4) [slashing] damage. 

Mounted Crossbow. RangedWeapon Attack: +5 to hit, range 100/400 ft., 1 target. Hit: 9 (2d6 + 2) [piercing] damage.

Opportunity Attack.  You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach