Let me lay out for you what I'm talking about. I finished the game the first time around with a party of four led by a 10/20 Swashbuckler/Cleric--excellent overall character. The second time through, I went semi-solo with a 12/18 Avenger-Fighter plus NPC Nalia. Very powerful duo, and I DID engage in open warfare with just about every monster I encountered. I then went solo for the first time, starting without equipment or XP, pure solo, with a 9/19 Stalker/Cleric. I was able to skip certain sections through Sanctuary/Stealth, able to do damage without risk to myself through Sanctuary/Blade Barrier and Sanctuary/Stealth/Turn Undead, did perfectly targetted offensive damage through casting "Holy Smite" on myself, and even had "Firestorm" and triple backstab available, as well as all the druid spells including the anti-mage "Insect Plague" and the combat defensive "Iron Skins," and also "Full Heal" and "Slay Living." Finally, I recently finished solo with a Blade but it was difficult.
I have also tried countless character combos, many even solo, with limited success. Based upon all that, I would say that the Avenger-Fighter and Stalker-Cleric are excellent for soloing, the F/M/T or Blade is okay, and of course the Swashbuckler-Cleric is really good although I haven't actually finished a solo game with it.