Ablative for Pathfinder 1st edition (Patreon)

Ablative is a creature ability you can add to monsters to make them shrink as they take damage. It’s designed for use with elementals, constructs, and slimes, but could apply to other creatures as well. If applies to creatures that primarily do weapons or natural weapon damage, it’s reduced damage output heavily counters its increased AC and accuracy. For creatures that use offenses not modified by the reduced size, it generally becomes more dangerous as it’s injured.

When an ablative creature has lost 1/3 or more of its HP, it becomes one size smaller until its HP total is healed to be over that threshold. This otherwise functions as reduce person.

When an ablative creature has lost 2/3 or more of its HP, it becomes one size smaller until its HP total is healed to be over that threshold. This also otherwise functions as reduce person, with the modifiers stacking with the first application.