OK, I'd shoot my balls off with the shotgun, causing the zombie to die of laughter...
Downside is I'd probably have to kill myself with the grenade, or possibly swallow the napalm... maybe both, whichever I feel like after shooting my balls off I guess, because I don't think I can predict the kind of mood I'd be in.
Alternatively, as this is most likely a video game situation I'd chuck the napalm on the zombie then shoot him because if explosive barrels have taught me anything it's that bullets = instant fire. So who needs matches?