Soul selection
I recommend Mordred as first 3rd-tier soul. The goal of new players should be to clear Advent Ultimates ASAP. Mordred has the best toolset for challenging difficult content with an underpowered team. In addition, her overdrive gauge extension EX skill is required for many Advent Ragnaroks.

Subsequent picks will depend on the needs of the primary team, which tends to track with element.
Healing (main): Cassiopeia or Andromeda
Type A DEF down (EX): Gawain
Type A ATK down (EX) / damage mitigation (main): Jeanne d’Arc
Type B ATK/DEF down (EX): D'Artagnan

Players should also pick up these souls for EX skills that are useful in certain encounters.
Mordred: overdrive gauge extension, combo attack down (from Asmund)
Cassiopeia: dispel enemy buff, cleanse debuffs (from Vivian)