While its very vulnerable to cards like Irohs Demonstration, I expect Dimir Midrange players to find slots for this one regardless
If your opponent loses the game after you declare attackers but before the melee trigger itself resolves (say, they were already at 3 life and someone Lightning Bolts them), the melee buff will still count that opponent as someone youve attacked this combat. What does matter is that you have to declare the attack for it to count towards the melee buff
They are doing a good job of making their new mechanics work, especially if they look really cool on the surface.
If you want to build a variation on Party Time with Tazri, I'd have you drop out the underperforming, overcosted cards that play better in sealed and draft and add in more removal and a few boardwipes
A great place to be in Modern
Then there are the game-impacting oversized cards, such as Vanguard cards