My Personal Opinion:
1st: Immolations: More quanta equals faster gameplay, more creatures.
2nd: Fog Shield (If you have a bad draw, one of these can save your life, and it only is one upgrade)
3rd: Lava Golems: +2 Damage each, not spectacular, but with 6 upped, it makes a difference.
4th: Horned Frog
5th: Vamp Dagger
I don't think the phoenix needs to be upped for this deck, and if you are at this stage of upgrading, I would go for a FG killer/improved PVP deck rather than upping the sparks.
(Also, if you plan on changing deck, all these cards I have chosen are quite commonly used in other decks, however apart from Immo rushes, there isn't a huge need for 6 Cremations, or 6 Lava Golems)