Strong, safe from harm, Polymorph form for a Familiar

Question

I reached the stage of the game in which I have access to two familiars and Imbue Familiar with Spell Ability. I want to use this combination to both improve my Action Economy and be able to cast supporting spells (like quickly reaching an ally with Revivify without repositioning myself). And since I'm already using a whole-day-long Polymorph on myself and Share Spells give it to familiar anyway I want to give my familiars some kind of evasive/immune to most things/impossible to target forms. My familiars are not supposed to fight using this new form, they will exclusively cast spells. New form is only meant to supercharge their defenses to untouchable degree, so I don't have to bother protecting them.

Initially I thought about Will'o'Wisp but it could only cast spells with verbal components, somatic components are no-go.

In case it matters: I'm a level 14 dual-progression caster with access to level 6 Sor/Wiz spells and level 7 Clr spells. I also invested in Incantatrix class so my buffs are basically permanent. I also "carry" on myself about 15 different buffs at all times, all of which are shared with my familiars so they are already a bit resilient.

DISCLAIMER: No Wish solutions, no Pun-Pun, no infinite loops allowed.

Answer 1

A few points of order:

• You are correct that, Rules-as-Written, a will-o'-wisp cannot cast spells with somatic components, since those require hands. Unfortunately, the only baseline familiars with hands are monkeys (in Dragons 277 and 351), which means imbue familiar with spell ability has some serious unwritten restrictions (probably not intended, though you never know). Personally, I'd rule that the familiar doesn't need hands, but you should get a definitive ruling on this from your DM.
• There are some (many) restrictions on polymorph. You should read both it and alter self carefully: notably, a polymorphed creature gets no (Su) abilities and no (Ex) qualities from its new form (which means will-o'-wisps would get neither spell immunity nor natural invisibility), they cannot be gaseous or incorporeal, and their hit points don't change. This means that the only meaningful defenses polymorph gives are a natural armor bonus and possibly a size bonus. (Or something like polymorphing into your enemy's mother, which is its own kind of defense, but a bit outside the scope of this question.)
• Polymorph, as a touch spell, is not normally a valid subject for Persistent Spell. You need to combine it with something like Reach Spell or Ocular Spell for it to work. I assume you've done so, but since it's not mentioned in the question I felt that I should clarify that point.

Three forms stand out to me:

• Young gold dragon: +13 NA and physical wings. Despite having claws rather than hands, dragons can explicitly perform somatic components (Draconomicon 6 – 7). Nothing in polymorph's text overwrites alter self's restriction that "[a] body with extra limbs does not allow you to make more attacks...than normal," so you don't get the impressive full attack routine of a dragon.
• War troll or firbolg: +14 NA for the troll, +13 for the firbolg. You don't get regeneration/fast healing, but do get daze attack/rock throwing. 31/16/29 physical stats for the troll, 36/12/23 for the firbolg. The war troll is a bit more defensive, but they're very similar.
• Thoon elder brain: +15 NA, but its flight isn't physical, so you're stuck scooching forwards with a 10 ft. move speed. Notably, the thoon elder brain's (Ex) Dual Action ability—which gives it two separate initiative counts, one for mental actions and one for physical—is inexplicably listed as a special attack, which means you do get it from polymorph. Were I the DM, I wouldn't allow this, but it's legal, RAW.
• A Fine creature of some sort: something like a hairy spider (Monsters of Faerûn) will have a high hide check, and even if your enemies spot it, they probably won't target it. Slap on nondetection so it doesn't glow like a Christmas tree to enemies with arcane sight.

None of these come close to being "untouchable," and the simple truth is that polymorph isn't the best defensive spell. Its power primarily comes from its offensive ability (that Firbolg has 36 Strength!) and versatility (which means less if you cast it at the beginning of the day). Other defenses, like greater mirror image (PHB II), greater blink (Spell Compendium), and greater invisibility will serve your familiars better, defensively.