You (normally) cannot use a creature as a weapon
Creatures are not objects, nor weapons, nor their bodies are (officially) defined as objects when they are dead. From Improvised Weapons:
Sometimes objects not crafted to be weapons nonetheless see use in combat. Because such objects are not designed for this use, any creature that uses an improvised weapon in combat is considered to be nonproficient with it and takes a –4 penalty on attack rolls made with that object. To determine the size category and appropriate damage for an improvised weapon, compare its relative size and damage potential to the weapon list to find a reasonable match. An improvised weapon scores a threat on a natural roll of 20 and deals double damage on a critical hit. An improvised thrown weapon has a range increment of 10 feet.
The feat Throw Anything simply removes the penalties of using an improvised weapon, but does not allow you to treat a creature as an object so it can be improvised as a weapon.
Except for Body Bludgeon
The Body Bludgeon Rage Power allows you to use a creature as an improvised weapon, but the feat has a specific mechanic that deals a fixed amount of damage for each creature size (tiny 1d6, small 1d8, medium 1d10 and so on). The feat makes you believe that it progresses normally with the creature size, following the Damage Dice Steps.
A large creature would cause 2d6 damage, a Huge creature would cause 2d6, a Gargantuan creature would cause 3d6, which is the maximum size category possible. Assuming you are big enough to be able to wield them as a weapon, which must be one size category smaller than you to be wielded as a two-handed weapon.
This means that the largest creature size you can wield as a weapon is a Gargantuan creature, assuming you are a Colossal creature yourself.
For Spellstrike to work, however, you will have to spend one turn casting your spell, and you cant cast with your off-hand because you are using both hands to wield the creature, then deliver it with a weapon attack on the next turn.
Spellcasting and Rage
Normally, you cannot cast spells while raging, but there are ways that allow you to do so:
- The Rage Prophet prestige class is able to cast some spells while in a rage.
- The bloodrager class is able to cast bloodrager spells while in a bloodrage.
- The a bloodrager with the Mad Magic feat will allow you to cast non-bloodrage spells while in your bloodrage.