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You level up assault and support by gaining ap. Roughly you will gain an alliance skill rank for every 2 alliance ranks. So you hit assault/support 8 at alliance rank 19, assault/support 9 at alliance rank 21, and assault/support 10 at alliance rank 23.
Edit: so to answer your question; wherever you earn the most ap.
Edited by FriedEggSandwich on September 26, 2015 11:42PM