Two, I am going to definitely have to disagree with you.
There is a reason why the Monk death blow character made it to the smack list:
Monk 2, Fighter 2, Psychic Warrior 2.
Str 16, Dex 14, Con 10, Int 10, Wis 16, Chr 8
Feats: Improved Unarmed Strike, Stunning Attack, Dodge (using the option for switching feats in OA), Mobility, Spring Attack, Ability Focus (FtLB), Combat Reflexes, Expert Tactician, Death Blow, Inertial Armor, Freeze the Lifeblood. If you don't allow the switching feats option in OA lose Ability focus.
Skills: max out tumble
Powers: (IDHMBWM - the one that you can activate as a free action that grants you +10 movement)
Equipment: Ki Straps (if you do not allow them to also work for FrLB, engineer them to only work for that ability), Potion of Expiditious Retreat, Potion of Greater Magic Fang (15th level caster), Potion of Bull's Strength, Potion of Cat's Grace, Potion of Owl's Wisdom, Masterwork Polearm of Choice, Full Plate w/gauntlets, Ring of Jumping.
Numbers: After Potion of Strength and Potion of Greater Magic Fang, your unarmed attack bonus (averaged) will be +13 (+4 (BAB), +4 (Str), +5 (Greater Magic Fang)). Your unarmed damage will be 1d6+9 (1d6 (base), +4 (Str), +5 (GMF)). Your unarmored AC will be 22 (10 (base), +4 (wis), +4 (Inertial Armor), +3 (Dex), +1 Dodge). Your Saves are Fortitude +7, Reflex +6, Will +7. Your FtLB DC is 24 with ability focus - a tough save for even the fighter or cleric (probably less so for the paladin). Your coup de grace save vs death is 35! No, I don't think even the fighters will make that one. Your unarmored speed can be 40' with the P.W. power. Moreover, if you want, just pick up potions of hiding and moving silently if you want to be the sneaky type doing what I suggested in my first post. However, you probably won't have to as your main danger - the spell casters can be taken out in the first round.
Tactics: Parade around as the warrior in heavy armor weilding the two handed weapon. You have CR, D, M, and SA to aid in this persona. Your AC will still be pretty good (20), and you have a decent attack bonus (+8 w/masterwork weapon) and can deal decent damage 1d10+4. The best thing is that you threaten a large area and have combat reflexes. Moreover, parading around this way, will not rouse the least bit of suspicion from even the more experienced players as you demonstrate a good number of feats in combat, have good tactics, and a good AC. If I were playing this I would offer a few cues, like perhaps being ungauntled one day and doing a punch when someone gets inside the polearm's threat range (rather than stepping back). Or maybe demonstrating the death blow when a cleric casts hold person or something similar - makes it more fun for the other players when they had a chance to learn about the BBEG ahead of time. Most of the potions all have hour magnitude durations so can be drank well ahead of time before the actual encounter against the PC's. I presume that whichever BBEG smack you choose will not attack the PC's at their most weakest (like when they are unarmored in bed or some such - as that would really suck for the PC's). Rather it will be at some climactic unexpected moment.
Moreover, perhaps you could have the guy directing this assassin like character somehow construct an encounter with the PC's against a rust monster - right before the climactic moment or perhaps at the moment. That way, your death blow monk just had his armor removed for him very quickly (rather than having to take it off).
You add in the rest.
Keep in mind that you still have several psionic abilities that I did not even mention, you can flurry (to great effect while buffed - and still get the deathblow attack), you have stunning attacks (take the feat rather than the monk ability - that way it works for character level rather than class level - granted you can use it less often), you don't have to drop the reach weapon when you attack unarmed, just release the two handed grip and then regrip during your turn if that would be most optimal (that way you can maximize CR). If it turns out that things are going bad for you, jump the heck out of dodge. With your +40 or so jump bonus you should be able to get to some pretty unreachable places relative to your party members. If you have RoF available pick up that hanglider device so that you can jump high then coast a ways or jump off a cliff or even jump up and then do a charge attack downwards gaining not only the +2 but additional +1 to attack from higher ground (and I think you may be able to include falling damage against your opponent).
A few other things of note - the GMF potion is made using create your own magic item, if you disallow this, instead get a +1 three section staff or polearm. This will significantly cut down on your ability to hit, but hey, thems the breaks - maybe you can secure a maximized version of some of the above potioins.
