Quite fun to use in PvE, too (when moebius blossom spamming ad infinitum gets tedious): combine with [skill]Assassin's Promise[/skill] as your hex, and you can usually repeat it as often as you actually need to.
I tend to avoid GPS when I can, though, since you can be instantly gimped by a single Shatter enchantment (unless you bring multiple enchants: shadow refuge is actually quite useful here, given the 8 sec recharge), and I lack nightfall, so my "I'm pissing around with AP" PvE build is usually
[skill]Assassin's Promise[/skill]
[skill]Mark of Instability[/skill] (since HotO nerf)
[skill]Black Lotus Strike[/skill] EDIT: don't believe the skill descrip here: this is now a lead, not an offhand

[skill]Wild Strike[/skill] (well, I needed an offhand, and I do love the stance removal)
[skill]Twisting Fangs[/skill]
[skill]Falling Spider[/skill]
[skill]Blades of Steel[/skill]
+ Self heal or res.
Which is decently spammable thanks to BLS and AP, though would be better off with BSS and FLS if I had nightfall and EOTN...but c'est la vie.
(having said all that, if anyone has improvement suggestions, I'm all ears: I'm not proud

)
Ultimately, S S: (if you haven't gathered this already) the best PvE elite you could ever have on an assassin...pretty much unarguably, is [skill]Moebius Strike[/skill], which essentially will let you spam dual attacks indefinitely. If you combine that with [skill]Death Blossom[/skill], the usual dual of choice, you can produce ludicrous rapid AoE damage for as long as your target stays alive. Also, sorry: slipping into general advice here.