[RPG] Arcane Mastery vs Spell Penetration which is better to consistently overcome SR


Spell penetration gives a +2 to overcome spell resistance.

Arcane Mastery allows you to take 10 on a caster level check to overcome spell resistance.

You have a 55% chance of getting a 10 or higher when rolling a d20.

And most monster SR's are fairly low… which feat will more consistently land?

Best Answer

What kind of campaign is it?

A monster's spell resistance typically equals its challenge rating +5 or, more often, +10. According to Table 3–2: Encounter Difficulty (Dungeon Master's Guide 49), most encounters (80%) will have an encounter level (EL) equal to or less than the four PCs' level. (The table assumes four PCs, all the same level.) With this in mind, in a typical by-the-book campaign for a caster whose caster level equals his effective character level, the feat Arcane Mastery (Complete Arcane 73) seems to this a player a better—but still not a great—choice: with already a 55% chance to best the spell resistance of at least 80% creatures encountered, taking 10 on the caster level check to overcome spell resistance means overcoming spell resistance, on average, all the time.

However, in a nonstandard campaign—as I suspect most are this late in the game's lifecycle—there's no way to know what the DM will throw at the PCs. An experienced DM knows when a lower-than-normal EL encounter is a deathtrap and when a higher-than-normal EL encounter is a walk in the park, despite the DMG's guidelines. Further, there's no way to anticipate what kind of challenges will be encountered when playing with a DM who makes the campaign Nintendo hard. In such cases, a better bet is the feat Spell Penetration (Player's Handbook 100), if only because, to overcome the tailored or optimized challenges in such DMs' campaigns, the PCs will need all the bonuses they can get. Taking the median isn't going to help.

Note: This reader recommends neither the feat Arcane Mastery nor the feat Spell Penetration for most casters; they should, instead, be finding ways to increase their caster levels and using spells with entries of Spell Resistance: No against creatures they identify as possessing huge amounts of spell resistance (or running away… or employing the Spell Compendium spell assay spell resistance (17) as this answer suggests). This reader does recommend the feat Arcane Mastery to casters (and especially warlocks) who have ready access to a number of dispel magic effects as being able to take 10 on caster level checks to dispel magical effects—rather than to overcome spell resistance—is certainly and frequently a thing.