See Spear Slicer for the Fast Strike rules.

I don't think 12 power for 6 mana should exist at rare. It might be okay at mythic: cf Armada Wurm. True, a simple Shock would kill this, but if they don't have that then this will kill in two hits, and red is the colour that's best at removing blockers.

Hmm... well, we'd kind of be assuming that this card comes with the line "When a creature is assigned to block or bed blocked by this, destroy it." I mean, there are creatures and scenarios that this wouldn't prove true, but the majority of the time, that's what will happen.

Hmm... it is tough to compare it to anything. Nova Chaser doesn't seem like a good analogue... too many fiddly differences.

Ah. What would you say if I compared this card to Rakdos Pit Dragon Alex? It doesn't get double strike until you reach hellbent, but then it can easily jump into a 12-power evasive creature. Does this card look more dangerous than that one?

You're right, it's really hard to compare this to anything. RPD needs hellbent, only costs 4 to cast, but then needs 3 mana on the turn you want it to be 12 power.