The highest grade in using alchemy is Rank 9. You can obtain this by mixing potions with ingredients together, but the simpler way is thus:
With the potions given from monster and boss drops, take the simpler ones (and even take empty Material and Mandragora to make extra) like Cyclone, Blaze, Toxin, or Volcano. Mix them together with only their likenesses (Cyclone + Cyclones, etc), and always choose to Mix the higher level with adding the lower levels in. It shouldn't take long before you end up with a Rank 9 potion. Keep your Heals at whatever level they come in unless your health has progressed higher, in which case you can boost it with mixing an empty Material in.