I googled it.
The trick is. Deposit your Pokemon (with correct item if needed). You then need to do a trade over the GTS (So find a Pokemon you can actually trade for) and trade for it. Then go to your deposited Poke and "Take it back", then it evolve
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Step1: Find a Pokemon on GTS that you can actually trade for first (So no one takes your poke you want to evolve). BUT Don't actually trade for it yet
Step2: Deposit your Poke with item if needed.
Step3: Trade over the GTS for the Pokemon you found (it can be anything remember, so search for like a diglett or something)
Step4: After trading on GTS, go to your Deposited Pokemon, and hit "Take it back". It should then evolve.
I actually found a Shiny diglett for a Palkia haha, so I was happy.