Normally exploding egg is removed from your inventory upon one use. However you can circumvent this. It's as simple as:

• Drop the egg
• Switch to other weapon just before explosion

That's it. After explosion ends you can do it again. The downside to this trick is that your other weapon will be overwritten with a Sling-shot. Also it's easy to forget and accidentally not switch during heat of the boss-fight.

Happy boss killing.