The two highest-tier Holy magic spells are Disintegration, which uses a massive blast of holy light that can destroy a mountain, and Melt Slash, which focuses the destructive light of Disintegration into a sword.
Earlier in the series, Replica Mamoru also demonstrated considerable destructive power with his own abilities, though his powers stem from a different source and may not count. In FINAL, however, Mamoru shows that they are potent enough to utterly vaporize his target if he dials the power up high enough.