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11:39
If we go the two-weapon fighting path, we can probably take Double Slice at level 7 d20pfsrd.com/feats/combat-feats/double-slice-combat
 
7 hours later…
18:31
@Baskakov_Dmitriy Shortsword will not cost a feat so you can get Weapon Focus instead, true. But it's slightly less damage and is piercing vs slashing for Sabre.
As I said before, Greatswords' bigger damage dice is more than offset by their penalty to-hit when multiattacking.
I would suggest that you don't worry about weapon-agnostic enhancements at this stage. Things like Enlarge Person, Lead Blades, Double Slice will affect all weapon-based builds similarly, so do not change the initial balance.
Actually...
Short Sword with Weapon Focus, 4x attacks at +7 (1d6+4) against AC21 is 18.2 DPR
Sawtooth Sabre with Exotic Weapon Proficiency, 4x attacks at +6 (1d8+4) against AC21 is 17.6 DPR
Greatswords with Weapon Focus, 2x attacks at +5 (2d6+6) against AC21 is 9.3 DPR
And then you Lead Blade, Enlarge and Double Slice your shortsword...
18:50
@Draco-S Thank you for pinging me this time. :)
Why do you use AC21 as a benchmark? How did you choose this exact AC?
It's an average AC of foes at tier 3-4 in 8-99 special.
I can check tier 7-8 instead if you wish.
8-99 is quite combat-heavy, so looks like a good benchmark.
So, going alphabetically, ACs are: 21, 24, 18 (incorporeal), 18 (incorporeal), 18, 20, 20, 27, 20, 23, 22, 19, 18, 21, 22, 20, 24, 19, 21, 18.
Average is still 21. Hmm.
Anyway. At level 7, with four limbs and four mwk shortswords, Weapon Focus and Double Slice you get four attacks at +11 (1d6+5) and 29.2 DPR
2x mwk Greatswords at +9 (2d6+7) net you a pathetic 16.8 DPR.
 
4 hours later…
23:21
@Draco-S The only possible and very real problem is DR...
Could you also give similar statistics to creatures' HP? We need some damage benchmark too to see if 29.2 per round is actually good.
As dealing 29.2 points of damage per round is the only thing that the Eidolon is doing in combat.
BTW, at level 7 you have 7 Evolution points if you are a Half-Elf.
So you can have 8 arms in total, adding 3 additional pairs of limbs, and getting Skilled (Use Magic Device), which, if you stack some other bonuses too, give allow your Eidolon to spend the first round casting Lead Blades on itself and waiting for your Enlarge Person to start working.
So, it's 29.2*2=58.4 before the size increases
Then it's going to get more than doubled again
Now we are talking. Hell yeah!
Any amount of DR cuts DPS to nearly 0, though.

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