Help With Campaign Writing?

This is sort of a multi-part question. I'm writing a campaign that I think is pretty cool plot-wise, but I'm not sure how to make it actually interesting to play. So there's three parts:

1. The Academy
2. The Collector
3. The Banishment

The Academy is a short(ish) segment designed to get the party to warm up to each other and get into the groove of the game. All the characters are enrolled (either by payment or by the yearly scholarship raffle the owners do) and get the same housing assignment. This is planned to go from levels 1 to 5 and is supposed to be just a little intro. How do I make this part fun?

The Collector is the main part of the story. A mysterious figure contacts the party and enlists them with a job. He wants help collecting the pieces of an ancient artifact for… "research." This segment ends with a twist villain (overused, but effective, I guess) as this figure uses the artifact to release an ancient being to destroy everything so that the world can be rebuilt. This villain grew up with a beautiful life, but it was ruined because of yet-to-be-determined reasons. He drops hints throughout that he wants a more beautiful world and would do anything to achieve it. He's basically just Lysandre from Pokémon X and Y. How do I make this section fun without it just being a series of fetch quests?

The Banishment happens when the ancient being is released (I'm rigging the fight so that right before the party wins the artifact is fully charged and activates so that they can see the ramifications of the madman's work). This segment is trying to recharge the artifact and get it to banish the being once and for all (or maybe unleash a different being that opposes the original and they have an epic kaiju battle with "Duel of the Fates" playing in the background while the party fights some scaled down version or minion). I don't have a big plan for this part, any tips?