Had to be placed on the target planet itself, so presumably not a long-ranged weapon unless they can teleport it onto Cadia or something.#1- Asguard Time Dilation field, set to 1 second for them = really long time from us (Whatever the maximum setting is)
Would take a good while to accomplish even if the cloaked ship wasn't detected. In the mean time, the Cadians begin evacuations and start thinking about revenge. Assuming the Necrons don't wake up and fix things of course - they might be able to.#2 - Fly in with a Stargate (under cloak), dial a Stargate near a black hole, and toss it at Cardia's sun.
That'd be on the bronze age civ's planet, not Cadia. Assuming the SGC, Tau'ri or whoever agreed to do this, and it worked, it could only harm Cadia itself if debris or radiation from the blast crossed however many AU separates Cadia from the other planet (never mind if the star's in the way). Cadia's a fortress world so the idea of orbital bombardments isn't new, and so most likely you'd just really piss them off .#3 - Naqadria Bomb + remote disabling of enemy Stargate. Hopefully, the Naqadria bomb will convert all the Naqadah on the planet into Naqadria and blow the planet to hell
As NecronLord said.#4 - Modified Replicator Disruptor Weapons, programed to affect Carbon molecular bonds (organics) / The Dakara Superweapon.
IIRC, it's partially based on Warp technology - the old Squats had it and I think Connor's mentioned the idea in an analysis or two as well."Plasma reactors" is a bit vague, but any fuel that can provide power for FTL and the weapons outputs 40k casually throws around should be stupendously exotic; think Hypermatter.