Spending Tempest Coins.

Since you play a lot of characters and you could always make profits off other events I'd take a good look at what's lacking and work from there. If your characters could do slightly better with more damage or you just like how the equips look, get the Tempest set. If you're worried that you might fail too many mastery books, get those for your most critical characters. Otherwise, just spend on rings; I wouldn't recommend shoulders too much since you don't know when Empress shoulderpads are going to be out and with people aiming for Tyrant equips there's likely to be a good number of players who'd aim for that.