While you have more than half Health, your Weapons have +30 » +60 » +90 Damage.

Your Weapons have +1 Damage. When you sell a small item, increase this bonus by +1 » +2 Damage.

Your Weapons have +2 » +4 » +6 Damage and Shield items have +2 » +4 » +6 Shield for each Friend you have.

When you use a Friend, your Weapons gain 3 » 6 Damage for the fight.

Double the Damage of your Large Weapons.

Your leftmost and rightmost Weapons have + Damage equal to 1x » 2x their value.

When you Burn, your Weapons gain +5 » +10 » +15 » +20 Damage for the fight.

Your Weapons have + Damage equal to your gold.

If you have exactly one Weapon, it has +5 » +10 Max Ammo.

...if it is also Aquatic, it has +25 » +50 Damage.

The first time you fall below half Health each fight, double the Damage of your leftmost Weapon for the fight.