Why would you want to use a weapon to kill a Sue that only has that effect in one out of every ten shots?

I know Acacia used curare to kill Sues on occasion, but I'm not familiar enough with LotR canon to be able to tell you whether that plant was canon to LotR. (I doubt it actually.) So, using a noncanon weapon might have been established as possible by TOS.

The important thing, though, is: would that weapon work in the fandom. For instance, in Harry Potter canon it has been explicitly said that electrical appliances can't be used at Hogwarts. Trying to kill a Sue at Hogwarts by throwing a toaster in her tub is not going to work, because canon says so.

When I first started writing missions in A-Team fandom I picked a handgun that would have been available in the time period the canon is set, but was not actually ever mentioned in canon. It's thus a noncanon weapon, but as a handgun, it could work in that canon.

As for making the death of a Sue an ironic echo, that's for agents with too much time on their hand. There's no rule that says you have to. If you want to kill the Sue quickly (with a neath shot through the head or simply breaking her neck) or slowly with time to run for cover (one in ten shots hitting the target?!) go for it.

By the way, you do not need permission to write missions to ask questions. Even if those questions are about things you would like to do in a mission.

First, have you permission yet?

On to the actual answer now...

I actually once brought a pulse rifle from a sci-fi continuum (Mass Effect) to kill two Stus (out of a trio of Stus and a Sue) in a Dark Fantasy continuum (Dragon Age), so it's possible to bring in a wildly noncanon weapon into a 'verse and get away with it. But I think it depends on what you're trying to do with the Stu in question. I killed the two Stus with the pulse rifle to give them a death not related to their canon of origin (Kingdom Hearts), mostly because it's an ironic echo of the fact that they both killed darkspawn in ways that would be more fitting in the Mortal Kombat continuum than they would even for the Dragon Age continuum.

Usually, we try to make the deaths some kind of riff/ironic echo played on the Sue/Stu the agents are killing. So keep that in mind when you do stuff.