This isnt a secret really it is more of a tip to train magic faster.
once you are able to use the spell shield, what you are gonna need is a good supply of matter ess and spirit restore potions for mana restoring.
the trick is to goto PL and get as many matter ess and srs and then head to TD entrance top left of map in PL this map is less than a sec cooldown for items that have cooldown.
once there get your menu set up to cast shield and also open your inventory and place the srs as near to the map flag as possible so you can click the srs fast once inside TD map and then click map to go back out to PL map to use your shield spell, in and out, in and out this sounds nasty i know.
keep repeating this process till you run out and have to restock.
dont worry about players or monsters attacking you as long as you do this fast.
hope this guide helps
PS: you can also do this with food like fruits or vegetables to make stuff with.