Raising the level cap from 80 to 85 will require a 100% completion with Guru's House (in which he will provide you with the Distorted Time Egg). This will usually require at least 2 full cycles of defense missions for Guru (roughly 14 missions). Being unable to enter Guru's House usually means that the player may have an alternate character that has completed Guru's house first (other than the character you are currently playing) that is not the max level. You will need to go back and raise the character that has completed Guru's House to the current max level for that account (to either 80, 85, 90, 95) before it will unlock the level caps for your alternate characters.
The level cap increase will apply to all characters on your save file.