You generally want to match your main weapon element with the rest of the team, as you don't want your soul to miss out on all of those weapon skills. You probably also don't want to run your water team against thunder at a meaningful difficulty, as eating a free +45% damage isn't fun.
You've got Gaia. If you've got Oberon, then a Gaia/Oberon/Cybele/Zephyrus lineup should at least see you through 3 months worth of thunder events (Gluttony reprint, Mastema reprint, and a raid event).

Sneak attack is...a type of buff to damage that's basically X% chance to do Y% more damage, but only if you have elemental advantage. SSR Cybele's page says that this does affect all attacks (normal, skills, burst), so it's very useful. But again, remember that it requires advantage. The probability and the damage bonus will vary from character to character.