Haven tactics.

Humans are primarily a defensive race; you need to develop their defense in every way possible, as gnomes have a weak attack.

It's unlikely to succeed; while gnomes have a weak attack, they have chaos magic and two types of priests. It will be especially difficult if they harass your rear with archers; then you will definitely have to attack. I really don't know how Haven can defeat a gnome; you need units that will force him to attack, such as true chaos units or necromancers with dark magic. It's difficult for a power-based race to defeat a gnome.