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This move always deals 20 HP damage. SonicBoom isn't affected by STAB, weakness, resistance, or any other factors. Ghost-type Pokemon are immune.
Everything bar Shedinja hits an HP stat of over 20 at level 100, and nigh everything reaches a level so far above 20 HP that SonicBoom is virtually useless. The only tier it would be useful in would be Little Cup, but it is banned there due to the comparatively massive damage it does to everything.
Dragon Rage always deals 40 damage.