The Old witch is definitely a support warcaster considering her low stats in SPD, DEF and ARM as well as her relatively few hit points. She makes up for this and can actually be a powerful caster with her high FOC stat and Great Power. If she is run with the right army list she can pose a major threat even in tournaments.
When choosing your army it is important to keep in mind that the Old Witch lends herself to a unique playstyle that is different from most other casters in Khador. Units with Pathfinder and 'Jacks with ranged attacks are good places to start.
For her battle group the Decimator, Destroyer and Behemoth really shine. Augury is a great boon to these models especially if you can plot some woods tactically during set up. The Kodiak is another great choice because of Pathfinder and his all around solid design. The Scrapjack is an entity of its own and as such will be covered a little later in this post.
The Units that mesh well with the Old Witch's playstyle are a bit more lenient because of Weald Secrets. I would recommend the Iron Fang Pikemen with Unit Attachment because of their solid stats before even becoming Pathfinders with Camoflage.
The other units that shine with her are the Kossite Woodsmen and the Kayazy Assassins with their Unit Attachment. The Kossites may seem like a nefariously bad unit statwise but what you want them for is the Ambush.
Widowmakers make a good defensive screen in a woods with the OW and are also a great offensive tool when the time calls for it. Finally: The Great Bears of Gallowswood fit into Khador most lists easily and it's no wonder why.
Solos are pretty important with the Old Witch and as a general rule they should be similarly set up like the rest of your army. Meaning Manhunters and Yuri the Axe. Kell Bailoch can prove to be invaluable as well.
The Scrapjack will act as your Warcaster for many frontline needs. As an Arc Node with two weapons with Reach and being able to have up to four focus allocated to it the Scrapjack is not often easily trifled with. Add in Avatar of Slaughter and there are few infantry units that can stand up to a charge from this 'Jack. Do remember to be careful though, the Scrapjack is survivable but only as survivable as a light warjack. Use Prowl and Murder of Crows to your advantage.
Now that you know where to start with an Old Witch army list it's important to know some basic tactics with her as well. The Old Witch is very much a command from behind the lines kind of caster. The Scrapjack alleviates many problems of leaving her in the safety of cover or woods.
The other 'Jacks you choose will be best served as an assault force that needs to be dealt with by your opponent. This will probably mean that they will be the first things to fall or at least the first things to get beat up. This is ok because the real threat comes from those quick pathfinding units and solos that you are going to have flanking around the sides. The Kayazy Assassins are very deadly at this job considering their Stealth and high DEF score (improved by Iron Flesh). Ambushing Kossites also have a tendancy to throw a ratchet into the sockets of the enemy's game plan.
Your Widowmakers will be invaluable once the oppenent deals with the 'Jacks you jam down his throat as they will be able to pick apart most anything that is left. they are also very good deterants to assassination attempts on the Witch. Kell's Silence will deal with any incorporeal flankers. If these defenders fail to do the job do not forget the spell Unseen Path which will get her out of sticky situations. Field of Talons will also deter most advancing threats that make it around to flank you.
Remember to be adaptable. If one tactic doesn't work then vary it to suit your needs. Some armies will be very unit heavy. In this case Greylords and Doom Reavers may be a better solution than IFP. Or if you're going up against Corrosion heavy or Fire heavy lists then try out the Assault Kommandos.
Remember to be Creative. Use units and models in ways that your opponent won't expect. Take advantage of your strengths and your opponents weaknesses.