Yea, it might be somewhat too tough, though the idea is that you should have several planets to set up serial ambushes against the thing, and it doesn't self-repair at all (though I may have missed a CanBeRepaired = false so ai engies might repair it). You're probably going to need to go through *multiple* full fleets so you'll need a pretty serious production line and resource backing to keep the ships coming.
Forcefields are pretty much a must if you're trying to protect something from it (notably command stations and heavy guns trying to do damage to it); I suppose by the low-power trick you mean getting the damage to switch to a less damaged ff so the repair-lockout-timer clears out for the first one faster? That's a good idea, though it makes this a bit trickier to balance. I balanced the avenger's offensive power to be able to work through a full set of full-cap mk I, mk II, and mkIII forcefields in something like a minute, given about 40 engineers rushing around inside the bubble trying to fix stuff (without me doing any micro tricks). So the idea is that you can buy yourself enough time on a wormhole blocker to get some more damage in before it slips through.
My suggestions would be:
1) lots of immune-to-forcefield attacking stuff to attack the heavy beam cannon IV modules underneath the avenger's shields; if you can take those out it gets a LOT less dangerous, aside from the death-nuke. I may need to buff the health on the actual modules to prevent this from being too easy, actually.
2) your own heavy beam cannons, preferably up to the IV, protected by some forcefields for survival, at a place where they'll get to shoot at the avenger for a while as it paths from wormhole to wormhole; it takes a while but the damage adds up, and these things have some fairly insane single-target attack power.
3) artillery golems do a nice amount of damage to those shields, and armored golems actually last a decent amount of time against the avenger's guns, at least they did in my test. Don't flinch when I say golems, you're going to need to play every card in your hand to get this thing (at least, you will if I didn't forget some cheap strategy).
