Increasing Spell Save DC

I have made a Lvl 15 wizard. I have already got both spell focus and greater spell focus. I have a 34 int(so +12) so if I cast a Lvl 5 spell it would be a spell save dc of 29(10+12+5+2). Is that good? Is there any other way to increase my dc? I know about demoralize through intimidate, and I need to beat the target's dc of (10+hitdice+wis mod). Is there any other way to lower someone's spell save dc?

According to this and the linked spreadsheet 29 is very good if you choose a spell that targets an enemie's weak save. So make sure that you always prepare different spells that target fort, ref and will and use your high Int to identify the enemies weakness.

Spell perfection is really the only thing I can think of you're missing but the ship might have already sailed for you on that one if you're level 15 as the pre-requisites are fairly steep.

Since you haven't mentioned which school of magic you have for SF/GSF, I'll just list a few general ones.

Bloody Tears and Jagged Smile (Pathfinder #71: Rasputin Must Die! pg. 75) increases the dc of spells with the fear descriptor by 4.

Silver (Alchemy Manual) used as a material component will increase the DC of illusion spells by 1.

Deific Obedience (Inner Sea Gods) Mahathallah grants +2 DC to illusion spells.

Dead Man's Headband (Ultimate Equipment) will increase the DC of any fear effect you create by 1. Note: it takes the headband slot, maybe see if you can combine it with a headband of int.

Draconic Perfume (Dragonslayer's Handbook) increases charm subschool DC by 2 (Rosewood and Myrrh) or compulsion subschool by 2 (Sulfur and Honeycomb).

Serpent Belt (Ultimate Equipment) increases poison by 4, whether your GM takes that to mean DC is up the them.

Elemental Focus/Greater (CRB) adds +1/+1 DC of spells with the energy type you select.

Sinner's Wage (Champions of Corruption) If used as a focus component for a charm or compulsion spell below 5th level, the target takes a -2 penalty to resist acting against its nature.

The trait(s) Lore seeker (campaign trait for ROTRL) lets you pick 3 spells to increase the DC by 1 each. The trait Eastern Mysteries (Guide to Pathfinder Society Organized Play) allows 1/day to increase a spell DC by 2.

Barbed Pentacle of Asmodeus (Gods and Magic) Increase the DC of spells that are augmented by writing by 1.

Rod of the Wayang (Ultimate Equipment) +1 DC to spells of the shadow subschool.

Not sure how to link them or I would, first post. Will update as I find/remember more.

While it doesn't increase the DC you should consider the Persistent Spell metamagic feat. This effectively forces an opponent to roll twice to save against your spells. It is a +2 spell level adjustment but at pretty much any level of opponent save forcing two rolls versus a DC is better odds than one roll against a DC 2 points higher.

For example, lets say you are facing a CR17 Marilith (Fort +25, Ref +18, Will +13), potentially a dangerous encounter. Identifying its weakness as being will you launch a level 7 Banishment spell at it with a DC of 29 (I assume abjuration isn't your spell focus school). It needs a 16 to save, so a 25% chance to resist.

If instead you cast Persistent Dismissal (the same spell slot level but DC27) it would need to roll 14+ and then, if it does, roll 14+ again. That reduces its chance to resist to about 12%, less than half the chance to pass the save than using Banishment.

I have made a Lvl 15 wizard. I have already got both spell focus and greater spell focus. I have a 34 int(so +12) so if I cast a Lvl 5 spell it would be a spell save dc of 29(10+12+5+2). Is that good? Is there any other way to increase my dc? I know about demoralize through intimidate, and I need to beat the target's dc of (10+hitdice+wis mod). Is there any other way to lower someone's spell save dc?

At this level you have access to various spells which will reduce an opponents saves.

Fear causes an enemy to be shaken even if they save and is at a level where you can quicken it.

Limited wish can impose a -7 penalty on an opponents next save but is expensive to use.

Frightful Aspect automatically causes enemies within 30' to be shaken and they become frightened the first time they hit you in melee.

Prediction of Failure causes an enemy to become permanently shaken and sickened on a failed save, a pass reduces it to 1 round per caster level. This is an effective -4.

Eyebite allows you to try and sicken any target once per round although the spell is unclear on what sort of action it is to do so.

Wall of Ectoplasm creates an enormous area wall which causes people who come close to it to become shaken.

Curse of Disgust can effectively cause creatures to become permanently sickened.

Since you haven't mentioned which school of magic you have for SF/GSF, I'll just list a few general ones.

Not sure how to link them or I would, first post. Will update as I find/remember more.

my main school is divination, but I have all sort of spells. I think my only divination spells are the ones I got to pick for being a divination caster. My opposition schools are illusion and enchantment but I dont mind having one or two spells from the school, its just probably not like 5th lvl or higher because that is an extra spell slot

Also to link you would do this
(any ".' are not suppose to be there I just needed it to not be a link)
(you dont need to link them, I just wanted to tell u how if you ever wanted to do links)
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Which schools of magic did you choose with spell focus and greater spell focus? Divination would be a terrible choice as the majority of the spells don't actually require saves.

Which schools of magic did you choose with spell focus and greater spell focus? Divination would be a terrible choice as the majority of the spells don't actually require saves.

Spell Focus(Evocation, Necromancy, and Transmutation)

Greater Spell Focus(Transmutation)

Reply to Ellioti's first post to see how to link.

Divination school is great for getting a jump on people, but the spells are not very useful offensively. Conjuration is pretty good at being save-independent, though you'll find plenty of very useful Conjurations that use saves. Most of what Evocation does requires a save - same for Enchantment. However, damage-dealing is generally considered the least effective use of a Wizard, and there are are lot of hard counters for Enchantment. Illusion has a few save-necessary things, although it's entirely usable without saves. Necromancy is a mixed bag - raising the dead is a no-save deal, but striking people down with curses and pestilences uses saves. Transmutation and Abjuration can be used fine without saves, although there are some offensive uses that require saves.

I'd most recommend Conjuration, Necromancy, and Evocation (in that order) to focus your DC boosting on. Conjuration is far and away the best school of magic. Necromancy is a powerful debuffing school with a few killing finishers. Evocation does damage.

If you want to further decrease saves/boost DCs, consider hiring a Witch with the Evil Eye hex, or taking Leadership. Alternatively, the Mesmerist's Stare does approximately the same debuff. There are certain bard archetypes which debuff as well.

If you're sufficiently immoral, try getting a Succubus to use its Profane Gift on you, for an extra +2 INT.

Are these good spells?:

Lvl 1: Feather Fall, Mage Armor, True Strike, Magic Missle, Technomancy, Shield, Ray of Enfeeblement, Air Bubble, Silent Image, Light, Unseen Servant, Keep Watch, Expeditious Retreat,Long Arm, Burning Hands

Lvl 2: Detect Thoughts, Knock, Scorching Ray, Acid Arrow, Rope Trick, Shatter

Lvl 3:Dispel Magic, Lightning Bolt, Seek Thoughts, Haste, Fireball, Protection from Energy

Lvl 4: Stoneskin, Dimension Door, Greater Invisibility, Scrying, Enervation

Lvl 5: Baleful Polymorph, Dominate Person, Prying Eyes, Feeblemind, Cone of Cold

Lvl 6: Chain Lightning , Disintegrate, True Seeing, Flesh to Stone

Lvl 7: Greater Scrying, Finger of Death, Banishment, Delayed Fireball, Reverse Gravity, Spell Turning

Lvl 8: Form of Dragon III, Trap the Soul, Moment of Prescience, Death Clutch