Here's what I would do in the chieftains position:
1) Get some powerful allies, the trolls from the troll moors, giants from the Spine of the World, tanarukka from hellgate keep, maybe some of the barbarian tribes living in the north.
2) Do not attack the human/dwarven strong points, attack them where they can't defend themselves properly, raid caravans, burn crops, destroy the roads leading to Silverymoon. The populace will soon start having hunger and empty money pouches. They will demand action on part of the militia.
3) Avoid open conflict and try to lure the human/dwarves into traps/ambushes.
4) Send human ambassadors (clerics of evil deities will do it) to human settlements to sow distrust, reporting to the common men that they are left to their own devices by their 'former' allies.
5) Be everywhere at once, a citystate is less likely to provide/send reinforcements to another city when it's own farms are under attack. By threatening multiple targets your antagonists have to spread out their forces, trying to defend them all. Then you can bring your forces to bear on a single target. Key to this is a strong mobile force so...
6) Try to acquire as many horses as you can get (raiding caravans cames in handy here). Even horses not trained for war can be used to quickly relocate infantry units, they just have to dismount before combat. And remind your troops, 'Horses are helpful, not food.'
The only drawback would be that orc warriors would demand all-out attacks and fast successes against their foes but that's where you - the DM - comes in.
~Marimmar