Well, with Life Orb:
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 252+ Def Slowbro: 26.39 - 31.21%
252+ Atk Life Orb Jirachi Zen Headbutt vs. 252 HP / 252+ Def Quagsire: 31.72 - 37.56%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 232 HP / 24 Def Slowbro: 35.98 - 42.41%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 4 Def Slowbro: 36.29 - 42.63%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 200 Def Slowking: 37.56 - 44.41%
+2 252+ Atk Life Orb Jirachi Zen Headbutt vs. 248 HP / 8 Def Heatran: 38.18 - 44.93%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 248 HP / 8 Def Victini: 37.96 - 45.16%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 0 Def Victini: 38.36 - 45.29%
+2 252+ Atk Life Orb Jirachi Zen Headbutt vs. 112 HP / 0 Def Heatran: 42.16 - 49.57%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 248 HP / 252+ Def Cresselia: 44.92 - 52.82%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 0 HP / 4 Def Victini: 45.45 - 53.66%
+2 252+ Atk Life Orb Jirachi Zen Headbutt vs. 0 HP / 0 Def Heatran: 45.82 - 53.86%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 184+ Def Eviolite Porygon2: 45.98 - 54.27%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 4 Def Slowking: 47.2 - 55.32%
Sure, Kyurem-B is priority-weak, but it still hits much harder than Jirachi and has all the coverage it needs. Hell, I'd argue mixed Kyurem-B has better coverage than Jirachi: Dragon/Fire/Electric has perfect coverage, while Jirachi's moveset is resisted by Heatran/Victini/Slowbro/Slowpoke/Starmie.
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 252+ Def Slowbro: 26.39 - 31.21%
252+ Atk Life Orb Jirachi Zen Headbutt vs. 252 HP / 252+ Def Quagsire: 31.72 - 37.56%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 232 HP / 24 Def Slowbro: 35.98 - 42.41%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 4 Def Slowbro: 36.29 - 42.63%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 200 Def Slowking: 37.56 - 44.41%
+2 252+ Atk Life Orb Jirachi Zen Headbutt vs. 248 HP / 8 Def Heatran: 38.18 - 44.93%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 248 HP / 8 Def Victini: 37.96 - 45.16%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 0 Def Victini: 38.36 - 45.29%
+2 252+ Atk Life Orb Jirachi Zen Headbutt vs. 112 HP / 0 Def Heatran: 42.16 - 49.57%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 248 HP / 252+ Def Cresselia: 44.92 - 52.82%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 0 HP / 4 Def Victini: 45.45 - 53.66%
+2 252+ Atk Life Orb Jirachi Zen Headbutt vs. 0 HP / 0 Def Heatran: 45.82 - 53.86%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 184+ Def Eviolite Porygon2: 45.98 - 54.27%
+2 252+ Atk Life Orb Jirachi Iron Head vs. 252 HP / 4 Def Slowking: 47.2 - 55.32%
Sure, Kyurem-B is priority-weak, but it still hits much harder than Jirachi and has all the coverage it needs. Hell, I'd argue mixed Kyurem-B has better coverage than Jirachi: Dragon/Fire/Electric has perfect coverage, while Jirachi's moveset is resisted by Heatran/Victini/Slowbro/Slowpoke/Starmie.