Assuming you know how to set up sm4shcommand open a fighters Animcmd/body, parse the motion folder, and set a attack Effect or whatever from 0x0 to 0x10 which will cause the hitbox to obliterate any foes. Then close the tab and save as. If u dont understand, there are other tuts to work with the program.
You could always reference Bayonetta's Final Smash script data to know what to do. Alternatively, giving an attack very high base knockback and knockback growth can do the job too; refer to my moveset edits to Wario-Man, where a fully charged Wario Waft has 200 BKB and 200 KBG when clean (can easily 1-hit KO, even a 1.7x-sized metal Bowser).