I like your setup and everything, But I think I'd drop the ranger classes to pick up more rogue or fighter, and go with a nice reach weapon, i.e. spear. granted the crit range isn't as good, the damage is little better, plus you can sneak attack at 10 feet away. If you have expertise and you don't kill them with the sneak attack, you can fight defensively at _5 ac, -5 to hit, and if your hasted you can really dish out some damage. Just hit him while ur 10 feet away with full attack action, then move back 5-10 feet with your hasted action. When he moves in, he will cross your threatened area and recieve a nice attack of oppurtinity, and if you manage to save and action and hold the action till he moves in, you can move back again when he moves towards you. Continuiting to do this will only allow them 1 attack per turn, cuz they have to use a move quivalent action to reach you. And your pretty much gauranteed 2 attacks, 1 on your round, and 1 attack of oppurtunity when he moves towards you.
Of course, If I really wanted to make a good assassin, I'd go all rogue and wizard, eventually taking Arcane Trickster. Use ranged Touch attack spells for your sneak attacks. At 10th level you could be pullin some massive attacks. If my memory serves me correctly, you'd be 3 rogue, 5 mage, 2 arcane trickster, and with the spell acid orb, you vcould do 7d6 acid damage, + I think 3d6 sneak attack acid damage, plus any other feets like point blank shot. and if you crit all that damage is doubled. ALSO, if they are flat footed, your ranged touch attack ignores all physical armor and natural armor, and they are denied all dex bonus, So the probability of them having above a 14 AC is very low. It will most likely always be 10 AC. With a high dex you'd be making your ranged touch attack at about a +10 to hit your targget, and of course if your a smart mage, your going to cast true strike first effectively bringing you up to +30 to hit, and again if your hasted, you can just lob another spell after that. Even with the meager +10 to hit, your only going to miss on a 1 and a roll of 4 at the most. And we aren't even going to go into maximizing.
oh the possiblities.............. *yum*