We are the precise center of the visible universe, but any two points in the universe which are far enough apart (many billions of light-years) are expanding away from one another at a speed faster than light.

Yeah, even with the technology we know the ETI has it's hard to imagine an attack which wouldn't be a civilization killer. It's already been mentioned using the gates to attack via strange matter, solar plasma or singularities - other possibilities are it throwing out a slug at 99.999% Lightspeed or a Black Hole's particle fountain. Depending on how the wormhole works allowing it to collapse may release energy in the manner of a decaying black hole, or it may be capable of opening a gate into Sol's heart, flooding transhumanity's population centers with solar fire whilst sending the star itself into Nova, or simply moving Sol somewhere else in the galaxy allowing the worlds of the solar system to fly off to freeze in the void. And that's before even getting to the freaky shit implied via FTL travel and Async abilities. An Alcubierre-Warp style device could produce a particle wavefront capable of burning the solar system to ash, the solar system could be locked into a CTC isolating us from causality, or we could be time-dilated to the heat death of the universe. Or it just fires a Gamma Ray Burst at us, retroactively triggered to hit the moment we become an issue. Oh, you didn't think that Pulsars and Magnetars were natural did you? Alternate vacuum states 'could' be a weapon against the ETI, but it could just as easily be something they can counter, or for that matter be something they live in. Essentially, the ETI is a civilization that is no longer subject to universal constants or laws – Gatecrashing has a few bit that could imply that the speed of light in vacuum is actually a variable, or that the Gate network could connect to alternate realities. So whatever happens is probably going to be very, very bad.