Heart of the Swarm Campaign- Char: Old Soldiers (Part 2)

After a few seconds, Zagara will highlight two facilities on your map- one to the far Southeast and the other at the center of the North side of Warfield's base.  These are locations where the Dominion have been experimenting on captured Zerg- destroying them will give Sarah one level each.  Happily, both of these locations are behind enemy command centers that you will need to tear down anyways.  The Terran command center to the Northwest has rare gold mineral deposits, which are especially rich and grant a particularly large amount of resources from each collection by drones.  When you capture the area, Zagara will send down four drones in a drop pod so that you can start gathering and generating units from there- not that you're liable to need to.

On the other end of Warfield's base in the Southeast, you can run across some large hauling vehicles- destroying these and tearing down the few Terran units and structures in the area will prompt five additional aberrations to pop up from the ground to aid you.

Generally speaking, no particular tactic is needed for this assault- the aberrations and any ranged backup you can provide should handle things easily.  This is a touch harder if you've not gained access to hydralisks, as many enemy units fly, but queens can suffice in a pinch, particularly if you have Sarah enhance them with Wild Surge.  Just keep rolling over defenders and supplying your assault force with new units and it shouldn't even matter that the Dominion calls down occasional nuclear strikes- particularly since these strikes target your forces, rather than your base, most of the time.  It’s not even remotely necessary to take over the mineral clusters in Warfield’s base- by the time you have captured them, your attacking force is most likely large and strong enough to take out the entire rest of the base in one long running combat (particularly with Sarah available to both dish out damage and potentially beef up your units).

Make sure that when you reach the final area of the base, you either have enough strength to destroy the main fortress immediately, or take out the construction buildings first- you can wind up losing a lot of lighter troops to the constant production of marines, goliaths, tanks, and wraiths if you don't blow up the factories, starports, and barracks on your way in.