When all your stars reach a certain level, e.g. every one is at least lv5 there is a hidden life bonus for your troops when attacking NPC's. What this means is a massive reduction in how much you will lose when attacking NPC's. For example if you attack a lv9 NPC with all lv9 stars your losses will be about 27,000 warriors out of 100,000. However lets say all stars are lv9 apart from 1, which is lv5, your losses will be about 32,000, and if that star is at 0, your losses could be up to 39,000.
In terms of archer rainbows when hitting lv10 NPC's, a full Lv 5 set will mean about 4,000 Archer losses, lv9 set will mean 3,000 losses and a lv12 set will mean about 2,000 losses. When you translate this into hundreds of attacks you will save thousands of troops.
What you need to do is make sure to have at least a lv9 star set and when upgrading always keep enough gems to make sure if you fail badly you have enough to get all pieces to level 9. Using the upgrading method shown on this site you can easily get your pieces back up to Lv 9 if you fail.
- Lv 5 Stars - 150% Life (Confirmed)
- Lv 9 Stars - 200% Life (Confirmed)
- Lv 12 Stars - 250% Life (Confirmed)
- Lv 14 Stars - 300% Life? (Guessing)
- Lv 15 Stars - 350% Life? (Guessing)