From my understanding there are 3 souls available at any time for each character high enough level to have unlocked 3. There are 4 saved skill sets for each character meaning within those 3 souls you can change around skills and save 4 very different sets of abilities, combining abilities from any of the 3 souls. Then, anytime outside of combat, you can freely switch between those 4 skill sets.
Switching either a saved skill set or a soul requires money and that the character be at a specific place, town, trainer, etc.
If the above is still how they are doing it, then a warrior character could have a strong tanking skill set, an off-tank/dps skill set and a pure DPS skill set with room for whatever combo of skills he might choose for soloing. but the restriction of only 3 souls at a time still applies.
We tend to become like the worst in those we oppose.