Sarah's Mind is a dungeon on the Heroic Path.
To access it, you must complete Valerie's Mind and choose to use Boost on her.
Next, when you encounter Sarah in the hallway, talk to Jane and then to Dr. Tiff in order to enter her mind. Which is literally on fire. Everything is on fire here. Everything. Including the enemies.
While you don't lose HP by being in here for too long, the enemies packs more of a punch compared to in Val's Mind and as such you should advance through it as soon as you can. That said, it's not a terribly hard dungeon.
Dungeon Walkthrough
In the first room, head north. You'll see what looks like a large room. Enter it.
In the second room, head to the bottom right corner and touch the Sigil there. This will thrust you into a boss battle, which is extremely tough compared to the boss in Val's Mind. Be prepared.
After defeating the boss, like in Val's mind, you get a few options.
Confusion
This will cause Sarah to enter her Needy personality at all times. While not a slave, you will be put on the Tempted Hero path.
Charm
She'll fall in love with you and put you on the Tempted Hero path. She won't count as your slave.
Suppression
If you wish for her to be your slave, this is your thing. Again, Tempted Hero path.
Grapple
Do you like tough chicks? Then this is the spell for you. She'll enter her Tough personality permanently. Don't use it if you wish to avoid the Tempted Hero path.
Boost
It'll make her free and also give her 50 more max HP. God knows she could use it.
List of Enemies
Fire Lady
WEAKNESS: Water
RESISTANCE: Fire
HP: Unknown (aprox. 200)
LOCATION: Sarah's Mind
SUGGESTED TEAM: Protagonist, Val, any
EXPERIENCE: Unknown
They hit fairly hard, but they're managable. Keep your HP up as they might spawn in sets of two.
Bear Claw
WEAKNESS: Water
RESISTANCE: Fire
HP: Unknown (aprox. 600)
LOCATION: Sarah's Mind
EXPERIENCE: Unknown
SUGGESTED TEAM: Protagonist, Val (third member if you want to)
This guy hits hard and he hits fast. Have the Protagonist keep your HP up at all times (keep your HP at 100+ as he can easily deal 80+ damage). Because Val probably lacks any hard hitting spells, just use your strongest spells on the boss. Toss in a Grapple with the Protagonist, if you can pull it off.
It shouldn't prove too hard if you have a decent level (or a good team; if you have access to Cassandra and Ashley, swap them in instead of Val).