Summon Monster + polymorph monster (aka beast shape)
tyrannosaurus [target creature]
N Gargantuan animal
Init +5; Senses low-light vision, scent; Perception +37
DEFENSE
AC 21, touch 7, flat-footed 20 (+1 Dex, +14 natural, –4 size)
hp 153 (18d8+72)
Fort +15, Ref +12, Will +10
OFFENSE
Speed 40 ft.
Melee bite +20 (4d6+22/19–20 plus grab)
Space 20 ft.; Reach 20 ft.
Special Attacks swallow whole (2d8+11, AC 17, hp 15)
STATISTICS
Str 32, Dex 13, Con 19, Int 2, Wis 15, Cha 10
Base Atk +13; CMB +28 (+32 grapple); CMD 39
Feats Bleeding Critical, Critical Focus, Diehard, Endurance, Improved Critical (bite), Improved Initiative, Iron Will, Run, Skill Focus (Perception)
Skills Perception +37; Racial Modifiers +8 Perception
SQ powerful bite
SPECIAL ABILITIES
Powerful Bite (Ex)
A tyrannosaurus applies twice its Strength modifier to bite damage.
http://www.d20pfsrd.com/bestiary/monster-listings/animals/dinosaur/tyrannosaurusPolymorph
Polymorph
School transmutation (polymorph); Level alchemist 5, sorcerer/wizard 5
CASTING
Casting Time 1 standard action
Components V, S, M (a piece of the creature whose form you choose)
EFFECT
Range touch
Target living creature touched
Duration 1 min/level (D)
Saving Throw Will negates (harmless); Spell Resistance yes (harmless)
Helpful Notes
+10 bonus on Disguise skill checks
Can't change into specific individuals
Your base speed changes to match that of the form you assume
With gained swim or burrow speed, you can breathe underwater/under-earth
Gain the natural attacks (and proficiency with them) of the creature using your STR/DEX for calculations.
Apply size modifiers to AC, Attack Bonus, CMD, Stealth, etc.
Unless the new form is humanoid, all gear merges into the form. Constant bonuses (except AC) remain. Activation items can't be activated. Material components are not accessible while merged into the form.
DESCRIPTION
This spell transforms a willing creature into an animal, humanoid or elemental of your choosing; the spell has no effect on unwilling creatures, nor can the creature being targeted by this spell influence the new form assumed (apart from conveying its wishes, if any, to you verbally).
If you use this spell to cause the target to take on the form of an animal or magical beast, the spell functions as beast shape II. If the form is that of an elemental, the spell functions as elemental body I. If the form is that of a humanoid, the spell functions as alter self. The subject may choose to resume its normal form as a full-round action; doing so ends the spell for that subject.
beast shape (II)
Beast Shape II
School transmutation (polymorph); Level alchemist 4, magus 4, sorcerer/wizard 4
DESCRIPTION
This spell functions as beast shape I, except that it also allows you to assume the form of a Tiny or Large creature of the animal type. If the form you assume has any of the following abilities, you gain the listed ability: climb 60 feet, fly 60 feet (good maneuverability), swim 60 feet, darkvision 60 feet, low-light vision, scent, grab, pounce, and trip.
Tiny animal: If the form you take is that of a Tiny animal, you gain a +4 size bonus to your Dexterity, a -2 penalty to your Strength, and a +1 natural armor bonus.
Large animal: If the form you take is that of a Large animal, you gain a +4 size bonus to your Strength, a -2 penalty to your Dexterity, and a +4 natural armor bonus.
if i read all that correctly.
you summon a nice pet dino (dont have to be a dino, i just like they tyrant king)
then you polymorph it. you polymorph it as beast shape II into a large animal (i would pick lion or tiger).
Large animal: If the form you take is that of a Large animal, you gain a +4 size bonus to your Strength, a -2 penalty to your Dexterity, and a +4 natural armor bonus
i do not see any text anywhere about negative effects from these poly/beast shapes. and i do not see that it ever mentions a starting size reference point.
and since your summoning in the first place, we should assume its gonna be Augmented, right?
base Str 32, Dex 13, Con 19, Int 2, Wis 15, Cha 10
augmented Str 36 Con 23
poly/beast Str 40 Dex 11
AC 21, touch 7, flat-footed 20 (+1 Dex, +14 natural, –4 size)
AC 24, touch 6, flat-footed 24 (+18 natural, –4 size) (might be able to argue less size penalty, but i wouldnt try too hard)
also 36 more HP.
ok, so here is the part when you tell me i am doing it wrong.
EDIT: i guess a better version (targeting Str) is using a Bison (base 27 str) ---> Girallon (37 Str)