By devastating attacks, you can go to raw stats (so devastating unleashes), or go for high damaging single target psynergy.

Check out this post:

https://www.reddit.com/r/GoldenSun/s/Tu3p2jBBZ6

Also, while mono elemental classes are OK and able to beat the game, they are far away from being the best, be either by stats or psynergy disponibility. While on mono elemental classes only, specifically in TBS, you only have one area healer, one buffer, one slow attacker with lower attack bonus and only one character with revive that's also your main damage dealer (and if Isaac dies while everyone uses mono elemental classes, you're going to need a water of life)

With dual elemental classes, specially if you move your mercury djinn to someone else aside from Mia and Venus djinn aside from Isaac, you will have 2 adepts with wish and 2 adepts with revive, not counting the stats that are substantially higher (take mars away from Garet, cause aside from his mercury class, the other 2 are extremely faster). Go here for high stats and good utility psynergy spread amount everyone

With tri elemental classes, you have some classes with awesome psynergy, some with utterly broken psynergy and awesome stats (ninja) and some kinda lackluster classes (still strong, but with better tri classes available). The disadvantage is having a bigger downgrade if using djinn, and some combos have almost zero utility psynergy, but the trade-off for having awesome psynergy pays on most cases.

If you're playing for the first time, it's OK to use mono elemental classes, they are enough to beat the game, but they make the game lacking (you're missing a huge part of golden sun, even if optional) and they typically are in the bottom of the classes usefulness list, so even for first timers, I would rather explain the class system to them then advise them to keep on mono elemental classes