Not taking the threat seriously regarding any real danger to his son, perhaps? The big guns Osborn has in his stable to send on a god-killing mission would be... Ares himself, and the Sentry. One can imagine how effective the latter would be against someone who can heighten and trigger anyone's fears.
ALEX: makes shadow puppet of a bunny on a wall SENTRY: OH NOES! The Void's come back and is going to kill EVERYONE!!!1! zooms uselessly into space
I must say, Norman is collecting quite a rogue's gallery of extremely powerful people he's unwisely threatened and pissed off. Morgana Le Fay, Namor, the Fantastic Four (not to mention Franklin Richards)... is his game plan to hope they all wipe each other out in the crossfire of trying to get him?