I ran an encounter in my on going shackled city campaign last night.. The party is currently going through Zenith Trajectory and I rewrote the koa toa monk/assassin as a skum ninja assassin. I had rewrote all the koa toa as skum which is why I chose the race. I have included his stat block below. Notice I had to modify the Skum stats to work due to their lack of Charisma. I did this by breaking down the Skum into a 15 point buy and restating them while trying to stay semi true to form.
:
Init +3
Defense
AC 19 Touch 14 FF 19
Hp 70
Fort 4 Ref 11 Wis 5
Resist Cold 10
Offense
Speed 20 ft. Swim 40 ft.
Melee Kama +11 (1d6+2), claw +8 (1d4+1), bite +8 (1d6+1)
Ranged Dagger +10 (1d4+2 plus poison)
SA Death Attack (DC 16), sneak attack 4d6
Statistics
Str. 14 Dex. 16 Con. 14 Int. 16 Wis. 10 Cha. 16
Base Attack +7 CMB +9 CMD 22
Feats: Multi Attack, Dodge, Weapon Finesse, Toughness, Improved Natural Armor
Skills: Acrobatics +13, Bluff +10, Climb +7, Craft (Alchemy) +8, Disable Device +10, Disguise +11, Escape Artist +10, Intimidate +16, linguistics +8, Perception +12, Sense Motive +7, Stealth +15, Swim +22, Survival +5, Use magic device +8
Party consists of 5 level 6 Characters.
Barbarian level 6
Paladin level 6
Drunken brute 1/Alchemist 5
Rogue 6
Monk 6
(I know it's an unusual party as the party Wizard and Cleric died)
Here are the breakdown of the rounds.
Round 1-3. Ninja uses stealth to hide from party in shadows. After 3 rounds he stealths to the Alchemist and makes a sneak attack for 4d6+*. 20 points of damage and alchemist fails fortitude save and dies. As a swift action he uses vanishing trick to disappear.
Round 4. The party is not sure what to do. With no arcane caster they are very defenseless against invisibility. Party readies actions and rolls perception checks to try and locate the ninja. The Ninja uses stealth to silently creep up the walls to hide in the ceiling shadows and begins watching enemy.
Round 5-7.
Rogue decides to check door for traps while everyone waits for the ninja to reappear with readied actions. Ninja makes sneak attack on monk, fails fortitude save and monk dies (rolled low). Barbarian attacks ninja with readied action.
Rounds 8+
Ninja uses shadow clones makes 3 mirror images. Combat ensues almost kills party rogue. 2 shadow clones die while ninja uses bleeding attack and a ki point to get an extra attack. He drops the rogue and does decent harm to the barbarian before finally falling in battle.
I know its a little light on info but it gives you an idea of how he performs. I was using vanishing trick and forgotten trick to do all the effects. By the end of the combat he was out of ki. Shadow clone was extremely useful for increasing the ninja's longevity in combat. Obviously the assassins death attack helped immensely however since the ninja can get this at level 10 and does it better, so consider it a look at things to come. As an NPC he made for a difficult encounter. However as a PC more care would need to be taken when using Ki. I could easily see going through all ki points in just 2 encounters if not very careful with ki usage. At least with the abilities chosen. While powerful I think it's a very well put together alternate class. I will say the rogue player felt like his character was weak comparably.
Knowing full well the brutality of Shackled City, and even remembering this exact fight... that looked especially rough haha.