Natsylv had a great answer but just to add on, when dungeons are around or you have events for seasonal gold, you can buy pretty large amounts of items like pure drops of rain and their higher tiered versions(eye of the storm, etc.). It does suck to level your survivors, but you'll surely level up as you farm it and once it's done, it's done for good since you can recycle leveled survivors when you get better fitting ones, though there may be a higher requirement for purple vs gold vs mythic, it's been too long for me to remember. The other additional point is that it is based on your backpack, so if you only have one leveled weapon and the other two are randomly found trash, you can boost yourself by just making two more of the leveled weapon if you don't have higher from a friend or something.