Each Twinmold has 20 health. Typically, the quickest way to kill Twinmold involves shooting the ends of their tails with arrows rather than using the Giant's Mask. Normal arrows deal 2 damage each; light arrows deal 4 damage each; and fire arrows deal 6 damage each to blue Twinmold, while ice arrows deal 6 damage each to red Twinmold.
Consistently hitting the tails can be rather difficult due to the size and shape of the hitbox:
The Twinmolds always move the same way at the beginning of the fight, and a bomb deals enough damage to cut out one fire arrow. Hold down left out of the cutscene and pull a bomb around when Link reaches the lower level of the platform (timing not precise). Throw the bomb at the blue Twinmold. Shoot its tail with a fire arrow early enough that it is not still invincible when the bomb blows up. From here, finish off blue Twinmold with two more fire arrows, then kill red Twinmold with four ice arrows.
Elemental arrows have significantly longer cooldowns when they hit something and their explosion animation plays. This animation doesn't play if an elemental arrow hits nothing or hits something that is sufficiently far away, so although Twinmold is harder to hit when it spawns farther away, you can fire elemental arrows faster and potentially finish the fight more quickly than if Twinmold spawns close by.
This TAS demonstrates sniping Twinmold with normal arrows. With good RNG, it can be possible to kill a Twinmold in only one cycle by hitting it with 10 arrow shots.
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.