I actually made a Ele/Psi for PvP reasons, but it worked just as well in PvE. With my build I could stack drain Psyche that actually drained the AVs in BAF. I always thought it was funny even with 0 end, they still used attack and cheated.
So I checked Siege. Here are his attacks, and the endurance costs of them:
Energy Punch: 7
Bone Smasher: 11
Whirling Hands: 18
Power Blast: 11
Explosive Blast: 20
(Incidentally, Power Blast's range was 310 feet). All of those attacks are well under the 53 points of endurance an AV recovers on a recovery tick. Suppose Siege was actually drained to zero somehow - between drain, recovery debuffs, and his own endurance burning. At zero, he can't attack. But its literally impossible to prevent him from getting endurance back at least occasionally. When he does, it will be a pretty big (in absolute numbers) tick of 53 end. But *visually* that will look pretty small - in fact my recollection is that because of the way the endurance bar is drawn the first tick can be mostly masked by the actual outline of the bar. Take a look at this video I made of a special no-stamina build (back around I8ish)
https://www.youtube.com/watch?v=tbM1lN1Xn-4Unfortunately this was back before I was making high res captures, but if you jump to near the end you can see what it looks like when you get down to the last end tick. Around 3:20 it actually looks like I get an attack off at zero end. And that attack is Eagle's Claw. That's actually me getting an attack off when I have only one tick left (and all my attacks are heavily slotted for end reduction). For me, that was around 7 end. It looks like zero because its obscured by the bar itself. For an AV, that same invisible last tick of end is 53 end, more than enough to get two or three attacks off.
I'm still recovering end ticks fairly quickly - about once every four seconds which is base recovery without stamina - but I think the visuals of me recovering 7 end ticks and using them a couple at a time for attacks is comparable to an AV recovering 53 end and burning them 7-20 at a time. In fact, Siege's attack costs relative to his end tick are actually better than my highly endred slotted attacks verses my recovery tick in that video.
Siege did slightly "cheat" in a different way I didn't originally realize. His version of Power Blast not only has immense range, it hits in an area of effect. But its typed ranged, not AoE. So AoE defenses wouldn't work against Siege's Power Blast even though its clearly an AoE. An AoE with the range of a cruise missile.