Multiple creatures are almost always stronger than a single creature unless N is to low. If you present him as a straight fight my money is on the PC's, but they are good ambushers. The PC's would still win I think, but not before one of them is severely injured or dead. They are better in packs. I would use two CR 5's.
Why not use Ranger/RMA. The skills bonuses match up with RMA, and the favored enemy bonus should give you more damage. You also get the extra TWF feats for free. If you use the guide archetype you don't even have to pick a favored enemy type. If they attack in their favored terrain they get bonuses to stealth IIRC, among other things.
1Alertness
1exotic weapon prof (sawtooth sabre)
2R TWF
3Weapon Focus
5skill focus (stealth) or Iron Will
6 RMA-Weapon Specialization (sawtooth sabre)
Enlarge person or Expeditious retreat would both be decent spells
Use the Guide Ranger archetype for Ranger's Focus.
The main problem is that most assassins, not just RMA, are designed to take out individuals who aren't even really ready for them. Not a whole group of uber-prepared adventurers.
That said, an RMA backed by a crowd controlling wizard would be pretty interesting. Also, if an RMA had to kill a whole group of PCs, my money would be on him using subterfuge to convince local authorities/thieves' guild/mob presence that the PCs need to be dealt with....and then trying to pick off individual PCs in the ensuing confrontations.
If an RMA just straight up appears out of nowhere and then tries to fight a group of PCs, he's doing his job wrong.
I think a pack of RMAs is a great way to create real fear in the PCs. Their big advantage is that they can initiate a prayer attack in the middle of combat. Now if they're alone, this can be easily disrupted, but toss 3 or 4 lower level RMAs against the party and have them maneuver the PCs around the battlefield until one is isolated. Have one of the RMAs start a prayer attack against that PCs while his buddies do their utmost to keep the rest of the party occupied. I think the suspense of knowing you've got 3 rounds to reach your fellow PC before he snuffs it would make for a pretty memorable encounter.
I think a pack of RMAs is a great way to create real fear in the PCs. Their big advantage is that they can initiate a prayer attack in the middle of combat. Now if they're alone, this can be easily disrupted, but toss 3 or 4 lower level RMAs against the party and have them maneuver the PCs around the battlefield until one is isolated. Have one of the RMAs start a prayer attack against that PCs while his buddies do their utmost to keep the rest of the party occupied. I think the suspense of knowing you've got 3 rounds to reach your fellow PC before he snuffs it would make for a pretty memorable encounter.
Though the PCs would really not have any knowledge of the three rounds unless they made some pretty nifty Knowledge checks or had seen it before. At the table itself...definitely some metagame tension.
I have actually just been working on building an RMA to throw against my players as well, and I found that Fighter is about the most powerful entrance. I know, weird, right? The couple extra d6s of sneak attack for going in with Rogue might be more significant at low levels (my RMA is level 15) than at high, but the increased chance to hit seems much more important.
I opted to dual-wield the sawtooth sabres (since they count as light weapons for TWF with the feat) and took just enough STR to give a little bonus to damage and focused on boosting DEX and CHA. My plan is to keep him greater invis'd to stack those sneak attacks on as much as possible. I also equiped him with bane weapons appropriate to the party. I mean, what assassin doesn't know his targets? The bane weapons obviate the need for the favored enemy of the ranger and the bonus feats for being fighter wash with the bonus feats from ranger.
Mind you, he won't be alone. I agree with the above posters, solo baddies are almost always weaker than an equivalent CR of lower level baddies. Action economy is the name of the game. You make these guys dangerous by giving them allies. Even a pack of CR 1 goblins provide flanking for sneak attacks.
I just played a great encounter from Curse of The Crimson Throne.
2 mantis assasins had hidden i a house and prepared the entire house as a fire trap. So while fighting the red mantis assasins we had a huge spreading fire to deal with.
I have actually just been working on building an RMA to throw against my players as well, and I found that Fighter is about the most powerful entrance. I know, weird, right? The couple extra d6s of sneak attack for going in with Rogue might be more significant at low levels (my RMA is level 15) than at high, but the increased chance to hit seems much more important.
I opted to dual-wield the sawtooth sabres (since they count as light weapons for TWF with the feat) and took just enough STR to give a little bonus to damage and focused on boosting DEX and CHA. My plan is to keep him greater invis'd to stack those sneak attacks on as much as possible. I also equiped him with bane weapons appropriate to the party. I mean, what assassin doesn't know his targets? The bane weapons obviate the need for the favored enemy of the ranger and the bonus feats for being fighter wash with the bonus feats from ranger.
Mind you, he won't be alone. I agree with the above posters, solo baddies are almost always weaker than an equivalent CR of lower level baddies. Action economy is the name of the game. You make these guys dangerous by giving them allies. Even a pack of CR 1 goblins provide flanking for sneak attacks.
The issue with bane weapons and targets change. From an immersion point of view you can't get magic weapons whenever you need them to match a specific creature. Strength is also better than dex for damage. A ranger focusing on a target with the guide variant will out damage a fighter most of the time. The ranger also gets more skills.
I would also suggest having the assassins run away if they start to get their butts kicked, and having a few low CR cronies if possible. I know they will die quickly, but it is free flanking, unless you have a summoning caster.
What about an Inquisitor entry to RMA? I haven't really thought it totally through, but Wraithstrike's comments on changing Bane weapons brought it to mind. Could work really well for an NPC designed for only one combat per day.