I suspect that Marcone has both a crash course in specific magic, power granted from a Fallen Angel (since Marcone didn't have a lot in the tank by himself), and Namshiel running co-pilot to assist.
Likely at the end of BG, Marcone had two issues. First, he wasn't it was worth trying to keep the eye from other interested parties. Second, throwing down with Harry isn't a sure thing (fortune seems to favor him). Third, killing Harry might spark conflict with Mab, the White Council (at least at that point), or even Odin.