Make sure you save inside the Gale Shrine BEFORE facing the Crimson Helm, because this game has a tendency to not pop achievements when they're supposed to, so you want a save file just before this boss, just in case you have to do it again.
This boss is a centaur-like samurai boss inside the Gale Shrine, which you can fight after you've received the Galestorm divine brush ability.
Avoid his charges and his flamethrower attacks, and keep attacking him until his armour falls off. When it does, his skeletal body sets itself on fire. Use a Galestorm (any direction) to remove the fire then attack like crazy (I recommend using a Rosary weapon as your main weapon) until he regains his armour. Repeat until he's dead.
After the battle, you'll get the Life Beads, Waka will take the Serpent Crystal.
The question "Would you like to save the game?" will appear on the screen. The achievement should pop at this point.
If the achievement doesn't pop at this point reload your save. If it does pop, click 'Of course!', and continue on with the game.