Take a drink of the beer... yes, 1.018 is on the high end, but I have medaled quite a few times with a FG of 1.018. If it is not too sweet, then I would not mess with it.
If it needs some help, I would force a small sample and see if you can drop a couple more points (raise temp, add yeast to the small sample, etc) and establish that there is indeed more fermentables left in the beer. If you cannot get it down, then don't stress the whole batch by forcing it.
If you do get it to drop a few points, then you may try the technique that worked on the whole batch, or try to krausen the batch. To krausen, I would rack the 1.018 beer off the yeast cake, make another similar or lighter beer, and put that on the yeast. Once that new batch gets rocking, I would grab some of the beer and top-crop yeast (if possible) and put that into the 1.018 beer and see if you can get the last few points to go down. The key is to have actively fermenting yeast, and not add priming sugar until the points have been dropped.
My fear with adding some S-05 or other yeast that is not present in your base beer, is that it may add character that is not desirable. If you can keep with the same yeast strain, your results will be more repeatable.