can you make a spell harder to resist by putting it in a higher spell slot?
for example suppose a level 15 caster wants to cast a harder to resist charm person.
can they put it in the slot of a 7th level spell to raise the DC? by 6 or 7 points?
Also please be aware of this FAQ:
Heighten Spell: How does this spell combine with other metamagic feats and using higher-level slots for lower-level spells?
Heighten Spell is worded poorly and can be confusing. It lets you use a higher-level spell slot for a spell, treating the spell as if it were naturally a higher level spell than the standard version. Unlike Still Spell, which always adds +1 to the level of the spell slot used for a spell, Heighten Spell lets you decide increase a spell's level anywhere from +1 to +9, using a spell slot that is that many spell levels higher than the normal spell.
The language implies that the heightened spell uses whatever spell level is used to prepare or cast it, but the rules text was inherited from 3.5 and doesn't take into account (1) the normal rule allowing you to prepare a spell with a higher-level spell slot, and (2) combining it with other metamagic feats.
For (1), having Heighten Spell doesn't mean any spell you cast with a higher-level slot is automatically heightened; you still have to make the decision to prepare or cast the spell an normal or heightened.
If you are a non-spontaneous caster (such as a cleric or wizard) who wants to prepare a lower-level spell in a higher-level slot, there is no reason not to use Heighten Spell on that spell (it doesn't cost you any extra time or any other game "currency").
If you are a spontaneous caster, heightening a spell when using a higher-level spell slot still increases the casting time, just like any other use of metamagic, so you have to weigh the benefits of either
• casting it normally using the higher-level slot
vs.
• increasing the casting time to cast it as a heightened spell and treat the spell as the level of the spell slot you're using.
Example A 10th-level sorcerer could cast fireball using a 3rd-, 4th-, or 5th-level spell slot, it would only be a standard action casting time, would count as a 3rd-level spell, and have a DC of 13 + Charisma bonus. If she had Heighten Spell and wanted to heighten it using a 4th- or 5th-level spell slot, it would have a full-round action casting time, but would count as a 4th- or 5th-level spell and have a DC of 14 + Cha bonus (for a 4th-level spell) or 15 + Cha bonus (for a 5th-level slot).For (2), you can't apply Heighten Spell to a spell at no cost: any increase to the effective spell level of the spell must be tracked and paid for by using a higher-level spell slot, above and beyond any other spell level increases from the other metamagic feats.
Example: A 15th-level wizard has Quicken Spell. If he prepares a quickened fireball, that requires a 7th-level spell slot (fireball 3rd level + quicken 4 levels). The spell's DC is still 13 + his Int bonus because it's still just a 3rd-level spell, even though it's in a 7th-level spell slot. If he also has Heighten Spell, the spell is not automatically heightened; it still counts as a 3rd-level spell and has the DC of a 3rd-level spell. If he wants to increase the quickened fireball's effective level with Heighten Spell, he needs to use an even higher level spell slot than the adjusted spell level from the Quicken Spell feat. Increasing the fireball's effective spell level by +1 (from 3rd to 4th) requires using a spell slot +1 level higher (in this case, an 8th-level spell slot instead of a 7th-level slot); increasing the fireball's effective spell level by +2 (from 3rd to 5th) requires using a spell slot +2 levels higher (in this case, a 9th-level spell slot instead of a 7th-level slot).Another way to look at (2) it is to add Heighten Spell first, then other metamagic feats. Continuing the above example, you'd first heighten the fireball to a 4th-level spell, then quicken it, which requires an 8th-level spell slot (fireball 4th level + quicken 4 levels). Or first heighten the fireball to a 5th-level spell, then quicken it, which requires a 9th-level spell slot (fireball 5th level + quicken 4 levels).
(Heighten Spell is a weak metamagic feat and has limited utility when combined with other metamagic feats.)
—Pathfinder Design Team, 06/10/13