The spell Demon’s Eternal Oblivion (DEO) in Ars Magica and the other variations for draining Might from supernatural creatures has an interaction with MR and spell levels which encourages taking the spell at low level and repeat casting. This means that a low Might opponent in Ars Magica can be dispatched easily by a low level spell cast with high penetration, which in turn somewhat trivializes lower Might creatures as opponents. It is not ideal to boost the creatures might just so it becomes more of a threat.
To nudge this balance, the Resilient Might personal power is suggested. It allows tailoring of how tough a Might creature is against those effects without just adding more Might.
Resilient Might (Personal Power)
Rego Vim, 0 Points, R: Personal, D: Sun, T: Individual
The creature is more resilient to Might draining powers and spells, and reduces the effective Might drained by such spells by (level of this spell -15).
(Base, +2 Sun, Constant)