Spoiler: Introduction
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Most 3E old-timers that chanced upon PF noticed the changes made to the polymorph subschool's workings. Specifically, those spells no longer replace the caster's own physical ability scores, merely giving a bonus to them now. As the Druid's Wild Shape is entirely based upon those spells, the class has been indirectly hit by the nerf-hammer. You can't really dump your own physical ability scores and still be a melee powerhouse quite like you did in 3E.

A lot of people liked those changes, as it mostly made sense. If you want to pose as the group's face-smasher, you actually have to invest in that aspect, or you won't live for very long.

It didn't come as a surprise, then, that many were baffled when the Synthesist Summoner archetype was published. The Synthesist can do in PF what the Druid (or any other polymorpher) could in 3E: dump it's physical ability scores and still rock at melee combat.

Snowbluff proposed a little fix here, and I believe it can work quite well. Basically, you just take the base form's physical stats, subtract a set value from it and gives the resulting number as a bonus to the character's own stats. In my opinion, the values can be adjusted a little more harshly than what was suggested by Snowbluff, though, by subtracting 12 or 13 from the Eidolon's base stats. The end result is quite nice: it aligns quite well with what can be expected from a Druid, gives the Synthesist a reason to invest in physical ability scores, and even means that the character can increase his physical stats and see that reflected in combat.

The end result is the following:

A more Pathfinder-ish Synthesist Summoner

Fused Eidolon: A synthesist summons the essence of a powerful outsider to meld with his own being. The synthesist wears the eidolon like translucent, living armor. The eidolon mimics all of the synthesist’s movements, and the synthesist perceives through the eidolon’s senses and speaks through its voice, as the two are now one creature. The synthesist directs all of the eidolon’s actions while fused, perceives through its senses, and speaks through its voice, as the two are now one creature.

While fused with his eidolon, the synthesist gains bonuses to his physical ability scores, as determined by his eidolon's base form. The synthesist gains the eidolon’s hit points as temporary hit points. When these hit points reach 0, the eidolon is killed and sent back to its home plane. The synthesist uses the eidolon’s base attack bonus, and gains the eidolon’s armor and natural armor bonuses and modifiers to ability scores. The synthesist also gains access to the eidolon’s special abilities and the eidolon’s evolutions. The synthesist is still limited to the eidolon’s maximum number of natural attacks. The eidolon has no skills or feats of its own. The eidolon must be at least the same size as the synthesist. The eidolon must have limbs for the synthesist to cast spells with somatic components. The eidolon’s temporary hit points can be restored with the rejuvenate eidolon spell.

While fused, the synthesist loses the benefits of his armor. He counts both as his original type and as an outsider for any effect related to type, whichever is worse for the synthesist. Spells such as banishment or dismissal work normally on the eidolon, but the synthesist is unaffected. Neither the synthesist nor his eidolon can be targeted separately, as they are fused into one creature. The synthesist and eidolon cannot take separate actions. While fused with his eidolon, the synthesist can use all of his own abilities and gear, except for his armor. In all other cases, this ability functions as the summoner’s normal eidolon ability (for example, the synthesist cannot use his summon monster ability while the eidolon is present). This ability replaces the class’s eidolon ability, bond senses, and life bond.

Base Form's Ability Score Adjustments

Quadruped: +2 Strength, +2 Dexterity.

Biped: +4 Strength.

Serpentine: +4 Dexterity.

Aquatic: +4 Strength.