You are kinda limited as an elf. Any base class outside of 1 level of rouge levels will give you 20% XP penalty, except wizard - which isn't going to help a great deal with melee rogue-ishness...
I reckon the best bet might be a 16th level Rogue (7 Rogue/9 Assasin?) with charisma. If you're allowed leadership, select a 14th level cleric, druid or wizard - have them buff you seven ways from sunday. Ring of spell storing with transfomation, righteous might or divine power.
In lieu of the cohort, take a high level of UMD and buff yourself via scrolls, staffs and wands. Expensive, but could be effective.
And now for something completely different:
The Woodlands Killing machine.
I'd consider Rogue 8

ruid 8.
6 feats - Multiattack, Improved Multiattack, Natural Spell, dodge, mobility, spring attack.
You can transform into large creatures with up to 8 HD. Take on Tiger form, giving you a pounce attack and reasonable stats.
Arguably, you can pounce in, getting 6 attacks - 1 bite, 2 claws, 2 rakes, 1 extra attack from haste. That's a lot of hitting. I say arguably, because AFAIK whether you can charge (to get the pounce) and spring out is a somewhat muddy issue.
You can buff yourself with natural attack improving items and/or spells, 'wild' armour and so on. If you can, buy a #day item of animal growth - preferably one another party member can use on you - as you won't be able to use it on yourself while transformed and can't cast it yet, but it's mean! (if a little questionable as well).
A slight problem is that greater dispels at this level will tear right through your spells like they weren't there. Diverse talents from complete divine would really help, if only you had the feats to spare!
Figure it feels quite 'Elfish', IMO. And at Rogue 9/Druid 9 - you'll be a lot meaner.
To twink this a little further:
Rogue 5

ruid 5: Assasin 3: Shapeshift enhancing PrC - 3.