A distinction should be made between the variances of 'easy', 'doable', 'complicated', 'efficient', etc.
DRK/Whatever and BLM/WHM, for most of their stint, are going to be waiting on the BLM's MP for healing instead of Nuking. This is why I said it would be a nightmare because it wouldn't be effective in an all-around leveling sense.
Even DRK+WHM, the WHM can break out a hammer and put that B+ skill to use while keeping both alive, a BLM blowing MP on cures might as well be a WHM, neh?
The ideal for a DRK in duo with anything but a PLD flows a little something like this:
1. The DRK pulls for the initial hate bonus because DRK is a hardier job than most realize.
2. The only debuffs that are particularly effective are those that make the mob deader faster.
3. The DRK receives support to stay alive while they kill the mob as quickly as possible (a dead mob deals no damage).
4. The partner remains useful whether the DRK needs healing or not.
5. The partner remains useful even when their MP runs out
6. The DRK always has available their emergency effectiveness (drains, stuns, blood weapon) because MP is not critical to their damage, and this is actually the purpose of #3.
A partner of any other melee job besides DNC (or /DNC) violates #6. Forcing the DRK to rely on their own self-support is going to result in MP stoppage or worse, and the DRK also cannot support a partner that cannot support themselves.
A BLM partner can fulfill #3 only at cost of other requirements, and falls flat on #4 if healing the DRK and #5 entirely.
A WHM partner covers all of the requirements listed.
A PUP partner covers everything with the right setup.
A DNC is a fantastic duo partner as long as they understand that the DRK will outdamage them and to debuff, keep TP for healing the DRK, and switch between Blood and Haste sambas as appropriate.
A PLD mixes things up in that they are the rare job that can take less damage and also keep hate over a rampaging DRK, so instead the PLD pulls and does their best to keep the mob off the DRK, but are still generally on healing support (breaking out a GS, however, makes them as much a monster as the DRK at low level).
What I'm getting at is that what DRK really needs in a duo partner is a healer than can do other things too, be it tank for them or melee at the same time, so they can not only kill as quickly as possible but also endurance, not relying on their own MP pool for support and such.
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