If you're using 6IV and 5IV parents plus Destiny Knot already, there isn't much else you can do. The knot will make 5 of the IVs pass down, but that last IV needs to roll a 31 on its own. It's all about luck.
Are you raising a mixed Salamence? If not, you don't need 6IV. And even if you are, nobody's really gonna notice if your lesser defense stat has a 20-something IV instead of 31.