Hey there.

As and adversary for my group of Rogue Traders (RT, Explorer, Void-Master) in their second Endeavour after "Into the Maw" I had the idea of a planet colonized by hereteks. They combined Mutant Psionics with Heretek implants. Of course the whole thing backfired and... well I do not want to spoil to much, just in case one of my players see this. They won´t fight this enemy alone, but with some low powered enemies between them.

The Stats (Modified Warp Witch)

WS: 38

BS: 28

S: 50

T: 40

Agi: 46

Int: 21

Per: 37

Will: 45

Fel: 13

Wounds: 18

Movement/Skills/Talents as Warp Witch (Rogue Trader Core Book P. 374) + Iron Jaw

Disciplines: Telekinesis (Representing the unholy control over nearby machines)

Techniques: Precision Telekinesis, Telekinetic Crush, Telekinetic Shield

Mutations: Aberration, Feels no Pain, Tough Hide (Spiderlike Appearance)

Cybernetics: Arms (Good, Scythe-Claws), Legs (Good, Spider-Legs)

Armour: Head 2, Body 5, Arm/Leg 4

Gear: Scythe-Claws (1d10+5, Pen 3)

Is this a suitable "Boss" for a Rank 1 Group of 3 Characters + probably some NSC Guards (which will be taken care of by numerous minions )?

I'd add Machine Trait (2 to 4), dump the voidsuit (that the normal wytch starts with) and go for subskin armour. Add a layer of natural voidskin in case they dump her into space.

Perhaps a Machinator Array (she could have ripped it out of a tech adept and incorporated it into her body)? You'll need the strength/toughness. This would remove the need for her to breathe and with all that metal she's not swimming anyway. Also mass x 3 is nice if you want to pin a fellow under a mechanical leg.

And don't forget Mechadendrites.

Specifically the Manipulator Mechadendrite (Common Quality, Back and Left Shoulder Mount) +20 to STR Tests, Can secure self to surroundings, use as a club (Primitive 1d5+2 I). No fine Manipulation. which is a big honking arm that can brace her and add 20 to strength tests when it is used. Nice for grappling.

One of those dendrites could be a really long tube that she injects into her dead foes and - by using it to interact with the corpse - re-animates it to fight at her side. Give it half characteristics and all the equipment and such the npc or pc had in life - make them pass a Fear Test if they wont to fight a comrade.

Add DISARM so she can have a chance to smack away melee weapons used against her - you can use the TK ability as a special effect - they come in for an attack and she DISARMS them by waving her hand - or takes the thing away and hits them with thei rown sword - either way it'll be a nasty surprise. This could protect her from the power weapons the party might have that cybernetic arms might not last against in a melee.

If your players have some high damage attacks consider Unnatural Toughness to x2 or x3 to increase the TB contribution to its damage resistance ability.

I reworked the stats. I do not want to go too heavy on my players, as they are not that heavily armed (Laserpistol + Monosword, Boltpistol + Powersword (Low WS) and Hellgun + Shock Staff). But I thought about placing something like an equalizer in the vicinity of this enemy. A Hellscorch-Pattern Plasmapistol. This thing could easily penetrate the Telekinetic Shield and do massive damage on the Witch.

Plasmapistol (Hellscorch-Pattern)
Class: Pistol, Range: 30 m, RoF: S/2/-, Dam: 1d10+9 E, Pen 9, Clip -, Overheat
Bound Demon (WP 33, Heart Seaker (+10 BS), Void Chill (1d10 Toughness Dam on Hit))

Spider Tech Witch

WS: 40
BS: 30
S: 60
T: 50
Agi: 40
Int: 20
Per: 40
WP: 45
Fel: 10

Wounds: 18

Movement 20/40/60/120

Skills
Awareness (Per), Climb (Str), Common Lore (Adeptus Mechanicus, Koronus Expanse) (Int), Dodge (Agi), Forbidden Lore (Adeptus Mechanicus, Mutants, Warp) (Int) +10, Intimidate (S) +10, Invocation (WP) +10, Psyniscience (Per), Secret Tongue (Tech) (Int), Speak Language (Low Gothic) (Int), Tech-Use (Int) +20

Talents
Dark Soul, Feedback Screech, Fearless, Jaded, Psy Rating 6, Resistance (Psychic Techniques), Iron Jaw, Hatred (Adeptus Mechanicus), Mechadendrite use (Utility), Mechanicus Implants, Machinator Array

Disciplines: Telekinesis (Representing the unholy control over nearby machines)

Techniques: Precision Telekinesis, Telekinetic Crush, Telekinetic Shield

Traits: Mutations (Aberration, Feels no Pain, Tough Hide) (Spiderlike Appearance), Machine 2, Machinator Array, Octaped (x4), Size (Hulking)

Cybernetics
Bionic Arm (Good) (S +10, +2 Armour (Arm)) (Scythe Claws)
Bionic Locomotion (Good) (+20 Jump, +2 Armour (Leg)) (Spider Legs)

Armour: Head 4, Body 4, Arm/Leg 6

Gear: Cybernetic Scythe-Claws (1d10+6, Pen 3), 2 Manipulator Dendrites (1d5+2 I, Primitive, +20 S)