OK, in no particular order, I have
Power Attack, Cleave, Great Cleave, Imp Bull Rush, Awesome Blow, Flyby Attack, Wingover, Lightning Reflexes, Epic Reflexes, Great Fortitude, Epic Fortitude, Iron Will, Epic Will, Improved Initiative, Superior Initiative, Dire Charge, Endurance, Epic Endurance, Improved Overrun, Diehard
I purposefully stayed away from Hover (it can't go into a gravity well, so it doesn't need to hover anywhere) and Damage Reduction/Fast Healing (since we wanted these to be fairly soft). If I'm wrong about Hover, I'd swap out Diehard for it.
Awesome Blow is contradictory to their preferred tactic of grabbing and dissolving victims, so I'd cut that.
My main problem is that I don't think the Power Attack feat tree is thematically appropriate for them. having such a diaphanous creature wailing away at opponents with might blows just feels wrong to me.
The problem is that cuts out a lot of the options.
Anyhow, I'm fine with the Improved Initiation, Superior Initiative and the Reflex and Will save-boosters (standard and Epic), plus Dire Charge is fine.
Let's see, that's seven.
Endurance, Epic Endurance and Diehard are OK, which brings it up to ten.
Flyby Attack is fine, and I suppose Run and Wingover won't hurt, making it thirteen.
I think we'd better give it Hover as its fourteenth feat, since it's the rules-approved way for it to full attack while flying without having a perfect manoeuvrability.
I am quite happy to give it a few Fast Healing feats, since Jellyfish may be fragile but they can recover from injuries remarkably well, although I agree Damage Reduction is inappropriate.
For the sake of argument, let's give it two Fast Healing feats, which adds up to sixteen.
For the remaining four, let's see.
Perfect Health seems a decent choice, and goes well with the Endurance/Diehard feats we've already given it.
Ability Focus (poison) will help it deal with its prey.
And to round it out, a couple of Epic Prowess to improve the attacks to +48. On the other hand, let's go for a single Epic Prowess plus Weapon Focus (tentacles) for the same result. I don't think there are any feats we want for it that require Weapon Focus as a prerequisite, but it doesn't hurt to add the versatility.
Altogether that is:
Feats: Ability Focus (poison), Diehard, Dire Charge, Endurance, Epic Endurance, Epic Prowess, Epic Reflexes, Epic Will, Fast Healing×2, Flyby Attack, Hover, Improved Initiation, Iron Will, Lightning Reflexes, Perfect Health, Run, Superior Initiative, Weapon Focus (tentacles), Wingover