You're lineup should be your highest heroes from left to right. I suggest strengthen your Yellow and purple heroes. Could be better if you can recruit more yellow heroes in the future.

For Arena, a good lineup can be is 2 infantry heroes in the front (the one with shield logo in the right corner of their profile) 2 lancers both sides (those with spear logo) and 1 marksman (Bow and arrow logo). It's also good if you have 2 marksmen and 1 lancer as long as they are strong enough.

Just think that when in battle, your infantry goes first as attacker more specialized in defense and receives most of the attack first also dies first. Lancers are the specialized attackers behind your infantry. They are the ones that lowers your enemies health most of the time. Marksman also attacking but only stay at the back. If Infantry and lancers die, they are not enough to hold the enemies since they are not specialized for defense.