Hunting for... Something to wield oversized weapons

[The Kool]

Alright, so I'm trying to put together a level 17 build in Pathfinder, with the ability to wield weapons one category larger than I actually am. I want to do this without a special race or dipping any class levels, if at all possible, and I have only first-party Paizo material to work with. So far I'm coming up empty. Can anyone lend a hand?

Here's what I'm aware of (and why it doesn't work):
Tiefling #16 (race)
Titan Fighter (level dip)
Titan Mauler (level dip)
Lighten Weapon (3rd party)

[Firechanter]

There's a relatively cheap item that helps a little, if only by reducing the penalties by 2. Something with bands or strips or wraps that go on the hilt.
But apart from that, you're still stuck with a Large Bastard Sword at best for which you need an exotic proficiency. So you'll be spending a feat and some gold for a whopping +2 average damage. Be still my beating heart. :6

Edit: scratch the feat, you can get that via ioun stone with wayfinder resonance. So just gold basically.

[The Kool]

Well I did run across the string, but it doesn't actually help the problem. As far as the weapon, I'm using the half-elf ancestral weapons trait for proficiency in the Butchering Axe, which achieves actually slightly better damage than an oversized Bastard Sword.

[Firechanter]

Well I did run across the string, but it doesn't actually help the problem. As far as the weapon, I'm using the half-elf ancestral weapons trait for proficiency in the Butchering Axe, which achieves actually slightly better damage than an oversized Bastard Sword.

Wow, I didn't even know this existed. Well, I think Paizo is making a point not to allow Oversized Weapon shenanigans in the way 3E did. What you can have is the Lead Blades spell. Buy a wand, UMD it, if necessary use a Wand Key Ring - presto. That should make it 4d6, so whether those 3.5 average damage are worth the investment is up to you.

[Kurald Galain]

https://www.d20pfsrd.com/magic-items/magic-weapons/specific-magic-weapons/effortless-lace/

[Florian]

Wow, I didn't even know this existed.

Hehe. Take a look at the Vexing Trickster Mesmerist archetype, especially the Surprise Strike. Now mix in a Butchering Axe and the Vital Strike feat chain. Add in Lead Blades, Enlarge Person, stir and mix well.

[exelsisxax]

https://www.d20pfsrd.com/magic-items/magic-weapons/specific-magic-weapons/effortless-lace/

This works for the large bastard sword, but can't be used to wield a large butchering axe, which would be effectively 3-handed.

[Andreaz]

Level 3 Primal Fury stance lets you count as a size category larger (and your weapons a further one for a total of 2 size increases). Without a dip it will cost you 3 feats, and you will gain a few goodies with it like scent and charge + full attack.

[Firechanter]

OP specified 1st-party material only, though. PoW is 3PP.

[Andreaz]

Well, that's lamentable.

[The Kool]

Wow, I didn't even know this existed. Well, I think Paizo is making a point not to allow Oversized Weapon shenanigans in the way 3E did. What you can have is the Lead Blades spell. Buy a wand, UMD it, if necessary use a Wand Key Ring - presto. That should make it 4d6, so whether those 3.5 average damage are worth the investment is up to you.

I have Synthesist already taking me to Huge size, so I can't stack another increase of my size (the DM ruled Enlarge Person does not stack with it, I am sad). Impact weapons are easier to get than Lead Blades and do not stack, per Paizo. Growing enchant is a nice catch from another book, then with two Vital Strike feats (I don't have the BAB for the third) I'm up to 36d6. A single increase in size here improves that to 48d6, an increase of 42 points, so I'd say it's worth it. I know a fighter or barbarian base could get higher damage by grabbing the fourth vital strike feat and being one of the Titan archetypes, but I'm trying to do this as a straight Synthesist.

So yeah, the only size increase left for me to get is something that lets me hold a weapon that's bigger than me. I've already got my size increase, a size increase on my weapon, and an effective size increase on my weapon.