This is by no means the only way to increase bond level, but it is one of the fastest ways i know. After unlocking all characters and you're at the stage where you need to find the 4 main ingredients for the LysAegis talent, go to the Tremel Tree and kite the 4 groups of bandits circling the pond to form 2 groups of 8 and keep repeating those battles, swapping pairs once 5 stars has beem reached between two characters. I would recommend only working on one pair at a time.