Clarion Conqueror is a 3/3 flyer that shuts off Ajani's planeswalker loyalty abilities and the activated ability on the flipped side of Fable of the Mirror-Breaker, all while blocking their small creatures profitably. It does not stop Ragavan, whose dash and combat trigger are not activated abilities, and it does not stop Guide of Souls, whose energy payment is a triggered ability. Teferi, Time Raveler is excellent since it restricts the opponent to casting spells only at sorcery speed, which strips away their instant-speed interaction, and its -3 can bounce a problematic Blood Moon or a key creature. Note that Clarion Conqueror also shuts off your own planeswalker abilities, so use Teferi's -3 before Conqueror lands. Endurance and Subtlety are weaker here since Boros has little graveyard synergy to punish, and Subtlety's trigger only handles creature and planeswalker spells, leaving burn, Blood Moon and Goblin Bombardment untouched, so trim some copies. Keep Force of Negation in since Blood Moon and Goblin Bombardment are both noncreature spells worth countering.