Go for the dual crystals. I picked up quite a few champs I wanted from them when I went hard for them a few months ago. They're more expensive than the regular crystals, but the chances of pulling specific champions from those classes go through the roof.
Hoard units and buy the year selector crystal on July 4.
Hoard units and buy the year selector crystal on July 4. This could work, Do you have an idea of how much units it'll cost