If I'm attacking a battlefield with Ezreal, and my opponent has a Yuumi, who's attacking/defending trigger resolves first?

In a combat showdown, the order in which triggered abilities resolve is determined by the Last-In, First-Out (LIFO) principle as applied to the chain. Because the defender adds their triggers to the chain after the attacker, the defender’s trigger will resolve first, and the attacker’s trigger will resolve last.

The Trigger Sequence

According to Rule 383.4 and Rule 442.1.b.1:

1. Placement on the Chain:

   • The Attacking player places their triggered abilities on the chain first.
   • The Defending player places their triggered abilities on the chain last.

2. Resolution Order (LIFO):

   • Because the defender’s triggered abilities are added to the chain after the attacker’s, the defending player's triggered ability resolves first.
   • The attacking player's triggered ability resolves second.

Note: I could not fully verify this ruling against the retrieved rules, as specific card text and the referenced flow document were not provided in the context.