it seems that all the mobs that will die from the magma/fire elemental *only* without being hurt at all by you, will not grant xp. just like if a hunter's pet kills something by itself the hunter doesnt get xp.

In short, mobs need to hit your shield spike to give xp. which means that you'll need to successfully block at least one attack from each mob before it dies.
Your clones will get that xp, it will work, but probably not as much as you think