Use a variety of colors. The bonuses for selecting troops with the same mana aren’t worth the frustration of waiting for one color of gem to appear so you can gain mana from matches.
Order your team carefully. Mana goes to characters closer to the top of your team first. Anyone lower on the list must wait for surges or the top characters to fill.
Select troops that have spells that work well with each other. Pair a troop that creates gems of a certain color with another troop that uses the same color of gem to boost damage output. If you have someone that does greater damage against opponents suffering from a status (e.g., hunter’s mark, webbed, burning) then add someone who inflicts that status.
Team bonuses are not percentage-based, so your low-level teams benefit disproportionately. They’re nice additions at higher levels but early on they’re extremely powerful.
Look for traits that add attack, defense, health, or magic, and load up your team with troops that meet the criteria for the bonus stats. There’s a good chance many of your selections will have traits that benefit the rest of the team.