OK, so the last post was a bit misleading...I didn't really discuss kaiju that much.
A game mechanism like Palladium's "Megadamage" rules works well for giant monsters and giant robots. 1 "megadamage" equals a hundred regular damage...when the typical person has abut 20 total damage they can take before dying, a single point of megadamage simply obliterates them...when a car measures a few hundred damage points, it only takes a few "megadamage" before it is rendered useless. The game system was actually written for Robotech, which uses giant robots and alien creatures that stand ten times taller than humans. But it has it's flaws as well, most notably that it is a linear system at base 100, 1 "mega" damage = 100 "regular" damage.
A similar game mechanism can be found in Aeon/Trinity from White Wolf. In that game system you have higher grade weapons and armour that deal in wound levels above human level. Everyone may have 7 health levels, but a "vehicle" class weapon simply kills them if they take a hit. If a person is capable of doing seven "personnel" damage in a single strike against a vehicle, they do a point of "vehicle" damage to it. In this system, the damage is a bit more incremental. Heavy weapons might do 4 damage with a hit (plus more with extra degrees of success), so it might be easier to do damage to a vehicle with a heavy weapon than with a dagger. I can't remember if it's an actual rule or if it was a house rule, but I used an extra degree of complexity to this system with "ship" damage. In the same way that it takes seven points of "personnel" damage to inflict a single point of damage on a vehicle, it takes seven points of "vehicle" damage to inflict a single point of damage on a ship. This is a linear system at base 7, 1 "ship" damage = 7 "vehicle" damage = 49 "personnel" damage.
Here's where Kaiju might come in.
A single hit from them obliterates a human, and most injuries from a human would simply be ignored by them. A bit like high level dragons in D&D, but instead of an arbitrary armour class or THACO, a damage reduction effect is in play. Maybe "Damage Reduction 10", where most weapons inflict 1d4 to 1d8 damage with a possible bonus of +1 to +2 due to strength. It's basically impossible to inflict 11 or more damage (which is enough to kill most people) in a single hit...and hits like this are negligible. But a decent hit of 15 or 20 damage (while enough to only kill 2 people), is suddenly dramatic to a large creature. It implies an exponential system, but it gets a bit messy with those big numbers. Bigger creatures could have damage division rather than damage reduction (or maybe both), but guess the higher number if hit point takes this into account.
If kaiju are so much bigger than humans, how can they be injured.
A good example exists in Pacific Rim, when one of the giant beasts is shot in the eye by a flare gun. This act may not specifically or critically injure the creature, but it has storyline significance.
I'm thinking that it might be an interesting path to develop a mechanism somewhere between the narrative and the strictly simulated.
Heavy weapons inflict automatic damage if they hit, Heavy armour absorbs damage automatically. Overkill weapons inflict massive automatic damage if they hit, and overkill armour absorbs similar levels of damage.
In Walkabout/FUBAR it takes 6 successes to permanently remove a character from existence, a regular weapon upgrades a single degree of success to two degrees, regular armour increases a single degree of absorption to two degrees (Walkabout allows relationships to push things further...with a possibility of instant kills if the situation plays directly to the active character's strengths). Perhaps Heavy Weapons in these systems might upgrade to 3 degrees of success (automatically eliminating a victim for a scene, or giving them a double penalty for an act); heavy armour would reduce the same level, thus pushing a triple degree of injury success to the most minor of wounds. Overkill weapons and armour magnify effects by five degrees of success (turning minor wounds into kill shots and vice versa). The kinds of weapons used against kaiju might magnify effects by ten degrees or more, in this way a character successfully hit by a "kaiju" class weapon is dead even if they are wearing "overkill" class armour that would normally render even the most savage wounds inconsequential.
If an attacker isn't planning to kill their victim, they can still invoke narrative effects like blinding the victim, confusing them or demoralising them (how do you demoralise a kaiju??)
I think I might need to explain things a bit clearer, but the general idea feels like it's heading in the right direction.