After reading a lot of complaining about the Synthesist, I decided it sounded like a fantastic idea for an evil NPC. Then I decided to Gestalt it to boot! Has anyone got ideas about what would be good with the Synthesist? I just picked fighter cause it was easy and I knew I could fill up the feat slots.
I just built a gnome alchemist bomber//synthesist for a gestalt game a few months back. I was trying to make Iron Man and that was the best way I found to do it. He was looking to be surprisingly effective by level 4 or so, and flying at will by level 5. I was grabbing the "extra discovery" feat as often as possible and not doing much with the meatsuit except grabbing the various defensive, skilled, and sense-boosting evolutions, and planning to get flight at 5th.
I'd give him a medium-sized (for better melee damage and disguise shenanigans), biped eidolon and max his intelligence, with maybe a 14 for charisma. Throw your gnome favored class bonus into "moar bombs!" and take the pyro and master tinkerer racial traits, boosting damage and enabling you to swing contraptions like ripsaw glaives and throw snag nets without penalty.
You might also want either the grenadier or clonemaster archetypes on your alchemist levels in order to get more effects on your weapons or the get-out-of-jail-free card that 8 levels of clonemaster bring. The hit to your bomb damage from clonemaster isn't a big deal, IMO, if you're using them to deliver stink bombs and other status effects rather than just damage. Saboteur is another option, enabling you to mix and match more bomb effects. I'd wave that silly "goblin only" restriction and give him rocket bombs as well.
You can give him more arms via discoveries if you choose and a tumor familiar - maybe a bat for the blindsense or a rhamphorynchus for a touch injection delivery system. Giving him gills via the synth levels is another good way to give him options for escape, hiding, and ambush that the PCs might not be able to easily counter and only costs 1 evolution point.
Pair him up with a necromancer stooge or 2 and give him the implant bomb feat for the occasional nasty surprise from exploding fast plague zombies or the like.
strictly speaking the synthesist is inferior to other summoners because they lose the class' outrageous action economy (and all the extra skills/feats)- they could make for a great npc/villain though.
it seems like you want to build a melee sythesist. fighter is probably not the best option since you lose armor use and rely on the eidolon's physical stats (meaning a lot of the actual person's base stats will be redundant, or he'll be completely ill suited as a fighter without the eidolon). you could build a Wis based monk with 14 (to maybe 16 tops) Cha, and be alright in combat even without E but have crazy good AC and deal much more damage with it. d8 is not the best hit dice, but you'll get a ton of bonus hp from E, have all good saves from the monk, and where a fighter would have his BAB reduced to E's the monk can still use flurry of blows to effectively have a full BAB.