If you have Gravity team support then Inferno can be very helpful as Gravity will boost its accuracy to 80%. This makes it more reliable than Will-o-wisp as a means to burn the foe. You can then use Hex along with Inferno to do a large amount of damage.
You can also use the move Telekinesis, which for three turns allows your attacks to hit regardless of accuracy or Evasion. However, this only works on one foe and Mismagius/Misdreavus will be the one to use it.
Telekinesis will likely be pretty risky as you are spending a turn which would be better spent attacking or using Nasty Plot/Calm Mind, but you don't need a teammate to set it up for you. I wouldn't suggest using this, although ingame it can be very useful.
Gravity requires team set up. If you are using a Gravity team, then Inferno will be worth using. It will provide type coverage, nice power, and a WoW effect that provides bulk and supports Hex.
If not using Gravity or ingame Telekinesis, I would not use Inferno.
(You should also use Inferno if doing Double/Triple Battles with a No-Guard ally)