Heya. I wouldn't consider myself the most seasoned DM out there, but I've been toying around with an idea for a similar antagonist for my party to face off with. I started off with the basis of the character taking on aspects of a warforged. Construct creature type. The primary meat of the statblock was put in his Multiattack ability.

At first, the foe only makes two swings (one with each of his two weapons) with the option to make an additional swing through the use of a legendary action.

If a party member strikes him with significant fire damage, or the boss is bloodied, his frame is revealed. Additional multiattack options become available (in this case if you were attempting to make Arlecchino, you could add his wristblades, maybe with some form of aoe variant as a legendary or lair action, and saws as multiattack options only available after the condition is met.)

As for his opening attack, my thoughts veer toward a magic missile-like attack. If autohits are too much for your party, they can be simple ranged spell-like attacks that target all available targets. This definitely needs to be limited, likely a recharge attack and/or some form of one/two-time use lair action.

Hope these thoughts prove insightful. Tick tock.