Typically, the quickest way to kill Twinmold involves shooting the ends of their tails with arrows rather than using the Giant's Mask. It should be noted that Fire Arrows do increased damage to Blue Twinmold, and Ice Arrows do increased damage to Red Twinmold.
Consistently hitting the tails can be rather difficult due to the size and shape of it's hitbox. Attached down below is a visual representation of what it looks like.
By performing mask storage using the Giant's Mask, you are able to take off the Giant's Mask without shrinking. This unrestricts our items and allows for a fast Twinmold fight with stick jumpslash Power Crouch Stabbing or Great Fairy Sword (if gotten early via SRM).
Using the Fierce Deity Mask, it is possible to kill Twinmold faster than using arrows.