Power up monsters with Angelmon of the same attribute
This is by far the most important part of a good power-up strategy. Using throwaway monsters to get experience is slow. Using Angelmon will give a much bigger XP boost. This is compounded if the Angelmon has the same attribute as the monster you’re powering up. The best power-up strategy is to get an Angelmon, evolve it with weak monsters obtained with Unknown Scrolls, and then use it as material for a monster of the same element.
Level up skills with monsters of the same name
If you get duplicate monsters, you may not want to keep them. Use them as material to get a healthy XP boost and to level up a skill at the same time. The same thing will happen when evolving. You can also increase a monster’s skill levels by using Devilmon as material during a power-up.
What monsters do you like to use for material? Let us know in the comments!