Advice.

Just about anything works in NM. But that build looks quite unwieldy for PvE.

Consider two things as an assassin:

1. Am I using my chain to the fullest efficiency?

Not really. [horns of the ox] is kinda bad in PvE because there is often a mob of monsters there to negate the knockdown clause. Also the damage sucks. Death Blossom is your most efficient attack, doing high target and AoE damage for low cost and recharge. Your goal should be to get to it as fast as you can, not waste time with lotus strike and horns of the ox. [Jagged strike] is also kinda lame. The one second recharge is wasted because you are busy with other attacks. A four second recharging attack generally suffices and provides a better bonus than bleeding. Final note: If monsters die before you finish your chain, your chain is too long. If you need a chain longer than four, your attack skills are too weak.

2. Will I survive?
Dodging damage is your best bet. Blocking and hit&run are your friends. Also, consider a better healing skill than [way of perfection]. A skill that heals when you need it is better than one that heals at random, especially since your chain has no use for an enchantment. Consider [heart of shadow] or [shadow sanctuary]. But honestly, having just one heal should be plenty.

Many will tell you that Mobius Strike builds are the best you can do:
[build=OwBi0xjMVmR8dw0w8WWDq/IAAA]
Since you may not have NF, [black mantis thrust] or [sneak attack] can replace golden fang strike. Asuran scan can also be switched out for any number of things.

For hard mode, it probably is the most damaging thing out there. Though, I often use the following flashing blades build, figuring that the damage from flashing blades adds up to about what I've lost from my Death blossom spam.

[build=OwBj0xfzITlZ+PHMSQ4Oce1gCAA]

Final note:
Not sure if you have Nightfall or not. If you do, you really should have [critical agility] on your bar.

Thank you for the responses - especially Smilingscar, exactly what I was looking for. Some explanation and 'why's.

I do have Nightfall but didn't run through the campaign with my assassin - I will be sure to do that for that skill however, is there any shortcut to getting it?

Mainly I was using [Horns of the Ox] as a sort of 'interrupt', useful on healers who are often stood on their own, but I will have a look for a better option if there is one.

Crit eye, crit agility, crit defences, way of perfection ( with your SA will give 30h back on every crit and only needs casting every 60 secs) , for the interrupt, beguiling haze with your SA at 10 thats a 7 sec daze, you can shadow to any target and interrupt it, add silencing daggers and you`ve got a 9 sec daze ...handy for upsetting all sorts of classes ...another daze option would be temple strike, gives an even longer daze and you could drop it in in place of a second offhand, esp handy against warr types as it also blinds

attacks, i use unsuspecting, wild and crit strike, not as much damage as blossom maybe and 2 secs long to recharge, but auto crit, heals with your perfection, gives energy, renews agility and defences...and as a build works quite well in PVP ( minus crit agility which can`t be used )

13 CA with crit eye gives 26% of a crit, so just over 1 in four normal strikes, crit strike is a dual auto crit..give you roughly 60h back and 14 energy

NONONO, less /D, it's for bad players. Less crit defense, it's what a prot monk is for. Run /W for broken SY spam. Palm strike owns the face off MS/DB because of armor ignoring damage plus snare. Respec your equips to 14, 15 or 16 dagger with +1 shadow and +1 crit only. Reflecting on what others say, if you want a KD chain, run trampling, falling lotus and twisting fangs, or just run twisting for deep wound so you can palm+twist. Also, finish him is a nice option as a final PvE skill, and working crit eye into a bar may improve energy management and ease of crit agi maintain.