Can you solo Sea Organs as 75MNK/NIN or /DNC? Obviously not UFOs.
I don't have DNC fully leved as a sub, and therefore haven't played around with MNK/DNC much, but I've farmed in sea on MNK/NIN with little trouble. The area with lower-tier Ul' mobs around the palace entrance is fairly easy for farming; really the only things to beware are linking multiple Aerns and targeting the first Xzomit in the trio (since they'll all link). Ul'Phuabo are DC-T in that area, but with evasion gear, the Om'Phuabo in the NW and NE reaches aren't too difficult, and have a much higher drop rate (and can drop multiple organs). I'd advise bringing plenty of Echo Drops, and possibly saving Dodge in case you need to turn around during their very potent Shock Spikes move.
If you're after Euvhi organs, the Aw'Euvhi in the Garden of Ru'Hmet can be farmed reliably in an evasion setup with MNK/NIN. I've found it safest to save TP and WS when the flower is in open-petal mode, where it has access to more damaging TP moves (most notably Vertical Cleave, which hurts with Counterstance up); this will send it back to closed-petal mode, which is safer. You'll also want to be carrying Echo Drops for these.
Since you brought up UFOs, MNK is a great job for low-manning them. Before I started Salvage, I would take some friends and duo/trio the Ul' and Om' respectively as practice for the chariots, since they have many similarities (charm move, 1K needles move, no strong physical WS). Mainly it's fun to just bring one of those huge things down. =)