Ancient Aquan Sentry

Antimagic Susceptibility. The armor is incapacitated while in the area of an antimagic field. If targeted by dispel magic, the armor must succeed on a Constitution saving throw against the caster's spell save DC or fall unconscious for 1 minute.

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

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

Lightning and Water Absorption. Whenever the construct is subjected to lightning or water damage, it takes no damage and instead regains a number of hit points equal to the lightning damage dealt.

Meltdown. If the construct starts its turn with 40 HP or fewer, it will trigger a self destruct that will trigger in 1 round.  It will deal its remaining HP as damage in a 30ft AoE around itself.

Illusion-Proof. The clockwork knight is immune to figments. Invisibility and other illusion spells that do not create figments affect the construct normally. 

Multiattack. The armor makes two melee attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 2) bludgeoning damage.

Relentless Programming. When the construct sees a creature that is unknown to it, not a construct, and not in a designated area, the construct marks the creature. The construct then pursues and attacks the marked creature until it or the creature is destroyed, or the construct is 60ft from where it is programmed to guard.

Magic Discharge (Recharge 5–6). Each creature in a 15 foot radius around the construct must make a DC 14 Dexterity saving throw. On a failure, a target takes 13 (3d8) magic damage and is stunned for 1 round.  This damage is lessened for every 5ft away from the construct the target is.
If the saving throw is successful, the target takes half the damage and isn't stunned.

Overclock (Recharge 5-6).  Until the end of its next turn, the construct magically gains a +2 bonus to its AC, has advantage on Dexterity saving throws, and can use its slam attack as a bonus action.  Each attack the construct makes during this time will also harm it for 3 (1d6) damage.

Parry.  The construct adds 3 to its AC against one melee attack that would hit it. To do so, the gladiator must see the attacker and be wielding a melee weapon.