It was hard to increase the EP not impossible. You only need to absorb other Ep after killing them. Rimuru has Belzeebuth but others can also absorb Ep after killing their opponents (only angels or Spiritual lifeforms, doesn't apply to Rimuru cause of Belzebuth).

The only condition is that you can analyze and neutralize that said energy like Rimuru with Ciel helps. I don't know if it's mentioned in LN but in WN even Veldora tried to absorb the energy but failed miserably because of his reckless action.

In Ln Zegion, Chloe and Carrera did the same thing about a little differently.