To defeat cave trial nr 5: Dont use Phoenix strike before you attacked the castle. The Goblin starts to run towards your castle if he gets attacked. Wait for a mass attack on the castle untill you got all your troops halfway the screen. Dont let any of your soldiers attack the goblin or get near him.
If the Goblin starts running try to gain some time and let the troops that get spawned by your castle run toward the enemy. Your Goblins are nice for this.