For Angel Legion – Idle RPG players, this guide will show you completing level 15 and level 30 of the Expedition has a chance to exchange for a common S-tier hero.
Some tips for the Expedition:
1. If you’re about to lose in a battle, you can withdraw immediately without any cost! (If you use the Certificate to get buff from Ares, the buff won’t disappear until you pass it.) This means that you can try as many times as you want to challenge a level. (If one of your heroes die accidently, you can also choose to withdraw and re-challenge the level.) 
2. You can skip the battles till level 9 or 12. Then start using manual combat by then to save up the hero’s energy to maximum.
3. Use some blue or purple commander items (especially blue bombs should be used to save resources because they are easy to produce in the cabin with cheap price and short time) to help you get through level 15-25. Make sure your energy is still close to full and your hero’s status is full.
4. We need to use healing Angel so that instead of bringing healing bombs we can bring other more useful commander items.
5. If one of your heroes dies in a battle, don’t revive her by giving your precious Certificate to Zeus. She will be reborn on the second day. 
6. Level 26-29 requires you to use the Certificates to strengthen your team depending on the situation. But please be careful not to use too much.
7. By the 30th level, you should still have 15~30 Certificates, all of which should be used to add buffs, and you should be strong enough to fight the BOSS. If you still have difficulty, don’t forget to use S commander items to get past it! 
Thank’s for posting this article, will definitely be keeping this as a bookmark.