Hm, the change between status value changes isn't really kept, but you can do that using status value change events.
Status values can automatically use game events when they are changed, e.g. also limiting this to negative changes, etc.
The events have access to the changes via variables, so you can use that to store the last change into an object variable on the combatant.
Or, alternatively, add a 'damaged' effect to mark the combatant to have received damage, e.g. ending the effect after a second or so (in case that should only be a temporary flag to check).
Both options can be checked in the battle AI.