Beerus oneshots. Elder Kaioshin's statement need not refer to strength, since using the Steve Simmons translation of the anime and cross referencing with the Herms translation it would seem Elder Kaioshin was treating strength and bad nature as two separate attributes when saying Beerus wasn't as bad as Boo:
Episode 250
Time: 12m30s - 12m44s
Elder Kaioshin: "Long Long ago, y'know, there was this terribly strong, y'know, bad guy, y'know. Well, not so bad as this Majin Boo now."
The main part showcasing Beerus' strength is Toriyama heavily suggesting he can destroy the Kaioshinkai, of which comprises 10% of Universe 7's entire macrocosm according to the SEG. As for comparing that to Boo Arc characters, we have this statement from Elder Kaioshin suggesting that the Kaioshinkai is capable of enduring an all-out battle between Goku and Pure Boo, the former of which's SS3 power he should be aware of.