I just recently completely my 75 colorless souls. I was 2-boxing THF and WHM mostly, but sometimes I did it solo as THF with Apoc, Gnarled Horn, and Razed Ruins.
Brulo's fire spikes hurt. You won't be soloing him as a DD unless you are /whm or /dnc or using resist atmas. They do up to ~75 damage per hit with no resists. Koios is the easiest. Brulo also tends to drop 2 souls the most often in my experience.
Maere is easy, but he and Gamayun are just the most annoying because they take the longest to kill. Also, in my experience, Maere has more of a tendency to only drop 1 soul a lot, even with high TH, which is pretty annoying. His Nether Blast can still hurt too. So can Gamayun's Aerial Blast.
Chione is the hardest of the T2 VNMs, but still just a T2. If you don't have stun, you can either interrupt her casts with melee hits and eat the ice spikes, or you can turn to avoid the ice spikes and let her spells hit you. I actually died to her once as THF/WAR when I didn't bring my WHM because I was derping. Ogopogo is really easy now. Tidal Wave only does a few hundred with ilvl gear compared to the 2k+ it could do before. Biggest prob with Ulegeurand is that it seems to have the most competition.
Overall, I wouldn't kill the T3 VNMs with anything but THF. They can all be done on THF/DNC. Their hit rate is pretty much floored so your only worries are spikes (Brulo/Chione) and high damage abilities/spells. Making use of Violent Flourish, Curing Waltz 3, and maybe temp items will insure success. Remember, some of your temp items always refresh when you enter an abyssea zone (depending on your abyssites) so be sure to make use of them (Lucid Potion I, Dusty Elixir, Stalwart's Tonic, Catholicon). I would also highly recommend buying and using Champion's Tonic every time as well as buying Lucid Potion II and Lucid Elixir I just to be safe.
This is a little off-topic, but honestly, I found the best way to farm souls is to rotate areas. I would normally go to Altepa first, camp Koios, if he didn't upgrade my colorful, I'd immediately warp out and go to Ule for Chione. Kill Chione, warp out, go back to Koios. Rinse/repeat. If there was a group camping Chione, I'd toggle between Koios and Gamayun. I set my home point at Port Jeuno HP 1 so that when I warped back, I could buy another instant warp scroll and just run to Horst real quick. You should definitely have enough damage to camp two areas until your colorful upgrades. Normally after I killed Gamayun or Chione, I'd go back to Altepa and have to wait a few minutes for Koios to spawn. I prefer this over trying to camp the third one because I always know their Time of Death whether I get the claim or not.