If I have a HE unit Keeper of the Flame in play.

Then below is the action chain sequence

1. I take a uncancellable damage to destroy Attachment card A

2. The DE player use a Tactic card to let my Keeper of Flame -1 HP

3.I take the second uncancellable damage to destroy Attachment card B

After resolved, Do I only destroy Attachment B ?

or I still can destroy Attachment A and B, even my Keeper of Flame dead in resolve DE player's action ?

I believe you destroy Attachment A as well, since once something is on the stack (or chain), removal of the source of that effect doesn't prevent the effect from being applied.