For the shadow-master, give him the same Physical Immunity to magic that Ogres have. Or, actually, from a thematic perspective, give him full Physical Immunity with a Catch not easily done with magic (or the magic the Focused Practitioner uses anyway), or that takes a few turns to set up. Combine that with Channeling/Evocation using Shadow as an Element, and he'll be really scary.
For the servants, Inhuman Strength, Inhuman Speed, and Claws paired with decent combat skills always make for loads of damage that probably gets to take at least one turn before it dies. Give them Cloak of Shadows, Stealth, and have them attack from ambush, and it just gets nasty. Nasty enough you may want to skip out on the strength, actually. Hmmm, I can throw together a quick minion...
Aspects:
High Concept: Living Shadow
Other:
Killer In The Darkness
Skills:
Great: Fists, Stealth,
Good: Athletics, Endurance,
Fair: Alertness, Intimidate,
Average: Conviction, Presence,
Powers:
Claws [–1]
Cloak of Shadows [–1]
Inhuman Speed [–2]
Gaseous Form [–3]
Total: -7 Refresh
Stress:
Mental: OOO
Physical: OOOO
Social: OOO
Armor: 0.
Toss on Inhuman Strength if you want more punch, but Claws plus probably getting your PCs to defend at Mediocre should be plenty. If they seem overly tough, drop their skills all a rank, that oughtta soften 'em up. As an extra benefit, they provide an excellent justification for their master to know the PCs weaknesses as he can have them watch the Pcs more-or-less at will.