I did some dread cr run, and I would say you can duo up to loca
A REC priest/orc can tank indeed, make sure you take the heal over time spells, they help save alot of MP.
For DPS, any class would do, fighter or ranger or archer or mage, it depend on your personal favorite. I personally go with mage, they got highest atk afterall and their atk is ranged so it help to take out the dread altar faster on the way.
And my duo even used novice gears linked with 3s. So if you have enchanted gears there should be no problem with loca, as long as you have good priest/orc and maybe some pots. Take note though, when loca dropped to half HP he will use a STUN atk that hit very hard.
Asmo on the other hand, is completely different story, he has a very dangerous AOE atk that can wipe out your entire party (3k crit). So if you don't have high enchanted gears and high HP, don't even try Asmo, just reset and spam loca.
In summary : You can duo spam loca with a REC priest/orc to tank and a dps. For Asmo you need good gears and at least 3 ppl with a pro healer