Complete the campaign. If the objective is to destroy all enemies, keep your bugs together and make usage of all the items and spells/skills given to you at the time.
If the mission is to get to a certain place, try to execute haste (via item or Healer spell) on Maal or a flying bug; this makes the mission a lot simpler.
For Mission 8 - Hold the Line, hold the entire team back at the wall. Allow Maal to stay in his starting position and send the archer and mage to the right side of the wall, where ice spells allow enemies to cross. When enemies start coming up, use Maal to push them into the water. When the enemy Ice Mages start making the ice, wait until enemies start standing on them, and then use the mage's Heat or the item "Bomb" to destroy the ice, causing them to sink in the water. Remember to use the Healer's Healing Aura, Heal or the item Healing Dew whenever a comrade is in trouble.
For all missions, don't be afraid to let Maal fight; he has an exorbinant amount of health and has a very powerful melee again, along with the ability to push enemies to their death.