I already posted in the 1.30 thread about this card, but whatever
It's a fun card, and as I mentioned there and others have here, it overlaps the same niche as phoenix and crimson dragon to a degree. I'm just happy anytime we get new stuff. I do think that an angel is a bit weird in the fire element, but I can learn to live with it. The ability as worded allows for some neat tricks like playing momentum or whatever on it and then making it untargetable, which is a trick not possible with aether's immaterial creatures. This is fine, and the card can still be killed in all of the ways that immaterial creatures can (well, the two ways :p) as well as things like quanta denial and then removal.
Two questions/comments I didn't think of last night:
1) Does the ability make Seraph immune to area effects the way immaterial does (pandemonium, etc) or only single target ones (shockwave, rewind)
2) The ability should actually be reworded. I know the -intent- is to make the card untargetable on the opponent's next turn, but as currently worded, it just makes it untargetable on the turn you activate it, and it will wear off by the time your opponent takes their turn, so it's actually useless :p It should say '2 turns' or 'on opponent's next turn, seraph is untargetable' or something along those lines.