This is a Tron mirror, and Breath Weapon stays in the sideboard. Its 2 damage to each non-Dragon creature kills nothing relevant: Rooftop Percher is a 3/3, the prototyped Boulderbranch Golem is a 3/3, and the rest of their creatures are much larger. Ancient Grudge comes in because it can destroy their Expedition Map, Bonder's Ornament, Barrels of Blasting Jelly, or a Giant's Boulder before they use its sacrifice ability, denying mana fixing or removal. It can also hit their artifact creatures, and its flashback makes it a two-for-one. We trim one Moment's Peace since their game plan is slow and grindy and we'd rather have interaction. This matchup comes down to who assembles Tron and resolves threats first, so prioritize mana disruption and card advantage.