Also, sadly, only one body fused at a time. So it's either find the one best cohort or fine a creature that I can suck 36 Str off of.
1) Get a thrall that can cast Simulacrum (ideally as a SLA, but even still... you can drain their XP dry and replace them later...). (And before TML gets here: I heard Mirror Mephits make cool familiars.)
2) Fuse with some badassed creature. It can be your 2nd thrall, or just something you've bribed/dominated/etc. You can use Metamorphosis to change your type to match, so having the same type as you isn't really a restriction.
3) Have your thrall create a simulacrum of the fused being.
4) The simulacrum of the fused being (which we'll call SFB from now on) can either manifest Fusion itself, or at least use a Power Stone of Fusion. (Depending on what level you were when this was done.)
5) Get another big, uber monster whose abilities and/or attributes you want to steal.
6) Have the FSB manifest fusion on it.
7) Have your thrall create a simulacrum of the resulting fused being (which we'll call FSB2).
8) FSB2 now has all of the abilities of the first two creatures you fused with, and is still a manifester (albeit probably a weak one).
9) Repeat until you have your uber-simulacrum that you want to fuse with in the end. Note that at any point you can actually fuse with the FSB to boost its manifester level back up to half of yours again.
10 Realize that you can gain nearly every power in the entire game with this combination.
11) Dodge dice and DMGs hurled at your head when you try this in-game. 
Even better...assuming you are a high enough level.
Step 1: Astral Seed yourself while in fused form
Step 2: Put your XP into a thought bottle and place it near astral seed (you probably want to create your own demiplane (since youre likely a shaper) to store it in. Hell, you can even give it the accelerated time trait so this will all happen pretty much instantly.
Step 3: Kill yourself.
Step 4: You're astral seed revives you at 1 level lower, you take the thought bottle and regain your level.
Step 5: Repeat.
This is only superior to Phaedrus' tactic when assimilating cohorts/npcs since you wouldn't want to simulacrum them and get only half their levels.
Theoretically since you are an entirely new being that no longer relies on the fusion power to maintain yourself. Your cohort/thrall no longer exists and you should be able to attract a new thrall/cohort and basically have 17 levels in every class combination possible. Not to mention every monster ability if you do the simulacrum as well.
...why didn't I do this with Azrael you ask?...because its stupid...its basically another pun-pun.
EDIT: Its also debatable whether or not you could fuse with a simulacrum at all since its essentially an illusion and not a real, living, creature. Honestly, if you could, you might be better off having a dweomerkeeper as a cohort/thrall and using him to make ice assassins to fuse with.
DOUBLE EDIT: Also remember, if you want your fusion to last long enough to really do something with it you (or a cohort/thrall) has to be a metamind. Honestly, if you don't want to get too ridiculous with it just be a StP Erudite Metamind with a Initiate of the Seven Fold Veil cohort (that's what Azrael was originally). You're pretty well protected with your veils and a spell aimed AMF around you 24/7. Work the action economy properly with contingencies (including crafted ones), foresight/sense danger (psionic power MoE), eyes of the oracle, magic affiliation, etc and you are virtually invulnerable.
