A) You said you have Diamond, which they aren't obtainable in even via trading with Platinum.
B) They can't be used in online battles anyway since rotom reverts to normal form upon connecting to wifi or entering a union room.
C) I'm starting to feel lazy having been working on this post for the last two and a half hours.
Disregarding the conditional ability of the last ghost,
Mismagius is the fastest choice of the lot while still maintaining decent special defense and special attack. The combination of movepool and stats make Mismagius the most versatile of your choices. Mismagius does lack defense, unlike your other choices, and so will only be able to take special attacks well.
*A note that I forgot before: since Mismagius is a stone evolution, it can't relearn most of its attacks. You have to teach everything before evolving.. and if you want to change something later [after learning a certain combination doesn't suit you, for instance], you'll probably have to rebreed.
Mismagius' most useful moves [in my opinion]
- Will-o-Wisp
- Thunder Wave
- Toxic
- Confuse Ray
- Curse
- Perish Song
- Pain Split
- Mean Look
- Memento
- Destiny Bond
- Trick
- Calm Mind
- Susbtitute
- -
- Shadow Ball
- Ominous Wind
- Thunderbolt
- Charge Beam [70% Sp. Atk up]
- Energy Ball
Curse + Will-o-Wisp/Thunderwave - Again, most opponents will switch when you curse, which gives you a chance to hit the switch-in with status. Mismagius' defense stat is low, so the switch will likely be quite a physical threat. T-wave will kill their speed and give you the first strike to do as you please, while Will-o-Wisp will halve their attack stat. If you're feeling lucky, you could even throw the dice with confuse ray, or try to force an immediate switch with another curse [offset HP to be an odd number so the second curse won't kill you]
Calm Mind + Attacks - Calm mind will boost your special defense as well as special attack, which could turn a speed and special defense trained mage into a sweeper. Just be sure not to switch in on something with Thunderwave or that may have a surprise physical attack.
Substitute + Destiny Bond Substitute on a switch, then destiny bond on the turn your opponent destroys it... they'll be left with a hard choice of what to do next. Quite situational to pull off this combo, since your opponent will have to be faster than you after the switch... but having substitute and Destiny bond functioning individually will work fine with a set and just make for a handy ace up the sleeve.
Drifblim is a very odd one.. you may have noticed that none of the walls mentioned above have very good HP ratings, which they make up for with massive defenses.. The opposite holds true with Drifblim and it makes just as good of a wall as they do. Drifblim's ability allows it to attain a higher speed than Mismagius with very little speed training, but it's conditional and you'll have to use an expendable item for it to activate.
Drifblim's most useful attacks
- Hypnosis
- Thunder Wave
- Will-o-Wisp
- Toxic
- Memento
- Calm Mind
- Stockpile
- Substitute
- Baton Pass
- Haze
- -
- Shadow Ball
- Ominous Wind
- Silver Wind
- Psychic
- Thunderbolt
- Charge Beam
- Spit Up
Calm Mind + Attack will work like stated above.
Stat-up + Baton Pass fairly standard and doesn't sound like what you're looking for, but thought I should mention it.
Stockpile + Sitrus Berry Makes for a fairly reliable way to boost your defenses as well as activating your ability [Sitrus restores 30% instead of set 30 HP now]
Spit Up instead of Explosion if you manage 3 stockpiles, Spit up will be more powerful and based off of the higher special attack stat.. it also won't KO you, so you can still get off at least one sucker punch on whatever gets sent out [assuming you KO'd and that Unburden is active]. Will obviously only work on a set containing stockpile, but is more successful than explosion on such a set in my opinion.
Hypnosis + Will-o-Wisp will work like the above Hypnosis + status, but Thunderwave is somwhat risky as well as not really being needed here [assuming unburden activates, you should be faster]. Electivire is the main reason I say thunderwave could be somewhat risky.. not so much for being predicted, but just because it would make a good switchin to try and take down Drifblim.