How to blow open secret hole in Brume Tower if I ran out of explosive carrying enemies?

Your actual question is:

Can I open this hole if I have exhausted the spawn limit of explosive carrying enemies?

Your initial premise is incorrect. You can no longer truly exhaust enemies. By joining the Company of Champions covenant, most enemies^* will have unlimited respawn.

Join this covenant, blown up the wall, and leave the covenant if you do not like the increased difficulty.

^*: there are specific special enemies that do not respawn:

Joining this covenant will give all non-special enemies unlimited spawns for the duration you're in the covenant (special enemies such as the Aldia Warlock in Shrine of Amana or the necromancers in Huntsman's Copse will not respawn). Once you leave, all enemies will return to the spawning condition they had before you joined.