The following applies to Story mode only, health and damage in Arcade scales completely differently.
Headshot damage in TS2 deals 4.0 damage. Reaper Splitters have 4.5/9/13.5 HP. Scourge have 3/7/10. So using the 100% of life as the measurement is inaccurate. Not to mention shotguns per pellet deal 2.0 headshot damage (diminishes at longer distance) which is roughly half the damage.
Regular enemies have somewhere between 1.0 and 3.8 hitpoints usually, so headshots can overkill.
Stone Golems and Wood Golems are actually completely different characters than what you can select (they have a white portrait, but inherit their aztec story attributes). Same with sentry bot (which has a unique death animation) and more health.
Laser guns also deal 10.0 points of damage as headshot or 8.0 in the chest. (more damage than explosions). Weirdly enough, laser guns deal only 6 headshot damage if an AI is using it, and 4 chest damage. (you have 70/40/15 HP depending on difficulty, or double that on Robot Factory/Space Station)
Crossbows deal 6 damage on headshots if a player is using it (3 headshots will kill your teamate in story co-op instead of 4). AI deal 4 headshot damage as usual and slightly less chest shot damage.
Arcade damage on most of the pages is actually correct. Except the damage is decimal not percentage (% is not a number, technically speaking). Players also have 2.0 HP so converting the damage % into a decimal then doubling it will give you the correct damage input. For example: a gun dealing 25% damage actually deals 0.50 points of damage.
AI bots have various amounts of health, and a damage resistance based upon their star rank. They have health from 0.75 to 1.25 general, with additional health given in frantic and a slight bonus (~5%) in chilled strangely enough. Generally, high stamina enemies have more health, but the star modifier can make enemies with less stamina be effectively more durable to damage (Hector Baboso vs Harry Tipper as an example).
Handicap Formula: d(5/6)^h
d = normal damage taken
h = handicap number (+10 for example)
An example would be sniper rifle deals 8.0 damage on a headshot against a player with Handicap +10. The formula we would use is 8(5/6)^10=1.2920446631187658 damage. Remember that 2.0 is needed to kill a player.
Found Handicap data here
Handicaps in game memory, labeled what I know.