Jesus... Uh, I would advise you to build him with a higher wisdom, and anything that would reverse time so that he could just not make that deal again. It sounds really... horrible and disgusting.
Uh... yeah, so I'm trying to help, but if you want help with feats and such, i'd need to know less about fleshy, meaty protuberances and more about what tactical role/stratagems you intend to use.
Are you building him for combat, or more for skills.. are you planning on making him a catalyst to motivate the PCS? If so, give him a decent CHA and diplomacy, so that when he starts screaming "PLEASE GOD HELP ME!!" the PCs will be sympathetic and not just run the hell away from the were-crocodile man-freak who's covered in... excretions... and whatever else.
It would also be a shame if his situation was so miserable that the PCs take mercy on him and kill him outright. Eh.... it's kind of a joke, but on the other hand, i'm serious. If I ran into some guy with his problems, and I was some big sword tough guy with an int or wis of 8, I might just decide that the only logical, merciful thing to do was to end his suffering, then slaughter the witches responsible. That second part would be another quest for the PCs, but the former would be the end of your PC.
Anyway, it's good to have a PC character as a GM in some situations, but not in others - the most effective role of a DM controlled PC is that of the Binder. They know at least two of the PCs well, or get along well with them; that way, you can use your character to sway (but not force) the party in a direction, or motivation.
Anyway, as for the build, summoner/shaman is a tough build to make work. The two don't seem to have a lot of synergy, and your spells will be lackluster because you're splitting your classes. If you're using them sheerly to jive with that backstory, tweak the backstory a little, in my opinion, and pick one class or the other. Works just the same, but at least you'll also be able to fill a party mechanic, instead of being that guy whose eidolon gets slaughtered all the time (or if it's what it sounds like, you're playing a shaman/synthesist?) and whose spells are quickly outclassed by other members, or that only last 2 rounds because of the low caster levels.
If you are doing the synthesist thing, then I think I know where you're going with this build; self-buffing juggernaut build, physical stat dumps, high mental stats. In which case you've got another problem; your pc will kill everything, while the players sit around eating popcorn and waiting for the 5 attack-a-round freakbeast to slaughter everything. As party mechanics go, it will not ingratiate you to the players if yours outshines theirs in every fight, which is why the synthesist archetype got banned from PFS.
Anyway, if that *is* what you had planned, it's going to be tricky balancing out his tactical viability. On the one hand, your shaman levels will nerf your eidolon's power level. On the other hand, you've got a wider selection of self-buffs, which may make the two/three off levels worthwhile. Improved Shared Spells, obviously, is the go to feat of practically every summoner, but in the cas of synthesists, its usually extra evolutions and/or augmented summons, or arcane strikes(which you just toss onto your natural attacks as a swift action - though I think it only works with whatever arcane levels you possess rather than divine ones). Don't bother with metamagic feats, as you won't have the higher level spell slots to capitalize, unless you plan on leveling much more heavily on one side than the other. There aren't many ways to synergize divine/arcane hybrids, but there are a couple... still I'd need to know what archetypes you've got and what the purpose is of your build (or at least what's intended)
Ultimately just keep in mind that GMPCs should definitely not outshine the PCs, because it will only build some resentment, or cause them to get bored - likewise, if the story becomes all about fixing/helping you, it can quickly get really tiresome for the players. Not in short windows, but certainly if it's at length.
Well, buffs will be handy I suppose, and taking multiple spell classes means a lot of lower level spells rather than a few high level spells - but you'll have a lot of those spells on your buff list invalidated by the low durations.
Augment summoning/spell focus conjuration is pretty standard for summoner types, and handy. Extra evolutions are nice too, to beef up your critters, as they'll be lacking with the split evolution pool. But more than anything if you're planning to go ragemode melee, take die-hard and/or combat casting! If you tend to rage before you have expended your spells, combat casting, so that you wont waste spells if you need em in combat- and keep in mind, with your stats split up among your casting classes, you'll have a harder time on concentration checks than a pure caster. Otherwise or also, die-hard, because, hey, if you aren't dead, and the damage you've taken is below 0 but not higher than your constitution, you can still take actions (auto stabilize, but -1 hp for each standard action you take). Actions like healing, or mentally ordering your eidolons and summoned minions to shield you from a coup-de gras, or mentally control them to kill enemies etc. Anyway diehard just sounds cool, kinda fits the image of this grizzly crocman.
Still, combat casting, diehard (and also endurance as a req) may be too feat heavy to allow you to do other things. With multiple eidolons, id go more toward extra evolutions, and extra eidolon feats, or just build toward that spell sharing (improved spell sharing first - so that you can buff an eidolon and yourself at the same time, if its next to you) I've seen two versions of this feat: one is a teamwork feat that requires both you and the pet/eidolon to have it, but no other requisites... the other is standalone (your pet doesn't need it) but has a 10 spellcraft prereq >.< which is ridiculously high, but has the same effect as the first one. So... you'd have to be level 10 to get it. If that doesn't appeal, then build toward Distant Spell Link - that way you can use those "self" buffs on your eidolons at close range, instead of having to have them always clustered around you.
Again, the augment summoning line of feats is great, especially if you bring out the eidolons during a combat (they die, then you cast the summon eidolon spell, or if you just dont have them with you and you start out with that spell). +4 str/con? sure. Later on, you can get superior summoning, which makes those random mobs of d3/d4 critters get a +1 to their number. Helps for swarming.
Also, focused eidolon is almost as good as combat casting - +4 to concentration if you're getting that shield ally bonus. Stack it with combat casting and cast defensively: you'll almost never have to worry about losing a spell in melee.
On the shaman side of things.. well, they're a little more limited, unless you take familiar feats, but all the good ones require level 5 (ahem, improved familiar) so instead, you could always get more hexes. Only thing is, most of the hexes are exactly what they sound like - negatives, curses, debuffs. Healing hex is a good one to pick up, just because it's a free heal, once per day per ally, that doesn't count on your spell pool. Fortune is.. well, alright, but you use up a standard action just to allow an ally a reroll. They might like it, but they also might hate it if you could've cast a heal or a buff on them instead. Ward is also a decent one - but it can only get laid on one ally at a time. Also, it's a deflection ac bonus and a resistance save bonus, so it won't stack with some of your other buffs or their gear, and those are the two more common ac bonuses. At early levels, it'll really help if one of your allies is getting beat on though. Anyway, you see my point, more hexes, maybe not as good as more evolutions or buffed up summons, other than those two (ward, healing) hexes. Unless you wanna take more debuffs, in which case evil eye, cursed wound, whatever, go to town.
Hope I've helped.
Edit: forgot to mention - the downside to augment summoning: your durations for summon monster spells will be really low (level 3 = 3 rounds) whereas if you use summon eidolon (level 2 summmoner spell) its 1 minute per level, which is much longer, therefore more reliable. Then again, if you cast summon monster 2, only one of the critters summoned gets the buff, so... I donno, maybe not the best buy. Stick with the hexes -- more allies = more uses of youe healing hex, and more people that get buffs from aoe buff spells. Just beware that dropping like 6 extra critters onto the battlefield can reeeeally slow things down. Have all stats prepared for anything you plan to summon, with buffs calculated on the side and inadvance, so that your players won't lose their minds waiting for you to flip through the bestiary for whatever critter you may decide to summon. Stick to one or two kinds and don't do the others. Also, if you're planning on keeping the eidolons down to two instead of going for the full on tiny swarm, they will thank you. And stick to the summon one critter variety, rather than the d3 - it'll probably be less powerful, but it's also one or two less rolls/time spent describing actions every round of a fight.
Anyway, that's my 10 cents.