When you click on a skill to buy it, you will start with mana already in your bar. Do not activate all skills at once, just one at a time as you run out of mana again.

For example, you buy Heavenly Strike and use it until mana is no more, buy another skill. You now have two skills and more mana, once those skills get used up and 0 mana, buy another skill. Repeat this until you have used all mana every round and have activated all skills.