The Magic combat tutor (named Mikasi) is one of the three basic instructors found on a small hill outside of Lumbridge Castle.
Every 30 minutes, you can claim 30 air runes and 30 mind runes for free, regardless of your Magic level as long as there are none in your bank or inventory. You also need to not have claimed training arrows from the Ranged combat tutor in the last 30 minutes as well.
It is possible to claim more air and mind runes even if you are already own some. You must drop all the air and mind runes you own and then claim the runes from the Magic combat tutor on a different square. Otherwise, she will observe that there are some air and mind runes at your feet.
If you want to train Magic with these runes, casting Earth Strike with a Staff of earth is a good strategy. It gives more experience than Wind Strike, but you will still have enough runes for thirty spell casts since Earth Strike uses one air rune and one mind rune.
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