This guide is mostly for Reincarnation/Endless/Boundless players, so keep that in mind.
First off, elements are different types of damage, which are:
Fire: Deals 50% extra damage to Flesh (red bar), but -25% damage to Shields (blue bar), and -25% damage to Armor (yellow bar), i.e. with a base damage of 10, to Flesh a fire weapon would do 15 damage, but to Armor or Shields it would do 7.5 damage.
Corrosion: 50% extra damage to Armor, but -25% damage to Shields and -25% damage to Flesh, i.e. to Armor a Corrosion weapon with 10 base damage would deal 15 damage, but 7.5 to Shields or Flesh.
Lightning: 50% extra damage to Shields, but -25% damage to Flesh and -25% damage to Armor i.e. a 10 base damage Lightning weapon will deal 15 damage to Shields, but 7.5 to Armor or Flesh.
Each of these damage types has some chance to inflict an elemental effect, usually shown in the weapon card which is shown in your inventory/backpack. Here are the effects and what they do:
Burning: Inflicted through Fire damage, and deals 20% of the hit's damage every second, for 5 seconds, essentially doing full damage of the weapon over these 5 seconds. Damage is, of course, fire damage, so has the same advantages and disadvantages. Stronger Burning effects take priority over weaker ones, and weaker effects are applied as soon as the stronger one ends, with duration reduced by however long they weren't active.
Decay: Inflicted through Corrosion damage, and -50% enemy movement speed for 5 seconds.
Lightning: Enemies inflicted by Lightning take +10% damage from ALL sources, for 5 seconds.
This is the part where it gets stupidly powerful. Fusion effects happen when two different types of elemental effects are inflicted onto the enemy, and have extremely powerful effects. Fusion effects do not cancel out the effects that cause them, so as long as one of the effects is kept active, the Fusion can be renewed by causing the other.
Combustion: Inflicted when both Fire and Corrosion are inflicted onto the enemy, and instantly deals 200% damage of the shot that inflicted either Burning or Decay to start the fusion effect. It also deals 100% of that shot's damage to every enemy within a 5 meter radius of the initial enemy. This effect can be triggered every 0.12 seconds per enemy. Each enemy hit is also staggered so they stop attacking for a small duration.
Manipulation: This effect does NOT apply to elite or boss enemies. Manipulation is inflicted through inflicting both Lightning and Burning on one enemy, and that enemy will stop attacking you and will attack nearby enemies instead, or stop attacking altogether if there are no enemies nearby. This effect is 5 seconds long. Damage done is worthless, but at least you aren't being shot at :p
Miasma: This effect is caused by inflicting both Lightning and Decay on one enemy, and it will deal 9% of the maximum HP of that enemy every second, for 5 seconds. Miasma only does 0.3% of maximum HP to Elites, and 0.75% of max HP to bosses, however. Miasma can be stacked up to 9 times, each stack dealing damage separately. The damage is dealt to Shields or Armor first, and is doubled when it does.
There are quite a few ways to inflict Elemental and Fusion effects, the easiest ones are:
Painkiller blessing, which inflicts ALL elemental and fusion effects on an enemy when you inflict any elemental effect on an enemy. This effect has a 2 second cooldown per enemy, though. Painkiller also turns negatives into positives when effects are inflicted on the player, e.g. +50% movement speed from Decay, -10% damage taken from Lightning, heal 20% of the hit that caused Burning every second for 5 seconds, and so on.
Elemental share gemini, which is a Gemini inscription that shares elemental advantages and effects between weapons, e.g. having a Thunderstorm and Fire Tower will mean both weapons have a 20% chance to inflict Lightning, and a 50% chance to inflict Burning, and will both deal +50% damage to Shields and Flesh. The shared advantage does not qualify for elemental damage bonuses except for the effect the weapon already has, i.e. a Thunderstorm with a 50% Burning chance, and fire advantage, will NOT have bonus damage from Pyrodominance or Furious Flame, although the inflicted Burning will.
Elemental Weave, which is a scroll that causes elemental effects to be forcefully applied within 3 seconds of swapping weapons, so having a Fire damage weapon in one hand, and a Corrosion damage weapon in offhand will let you inflict Combustion reliably by just swapping and shooting once.
There are many other ways, but these are the strongest.
Li is extremely good with Fire damage and effects, and has many ascensions boosting it or getting benefits from it, e.g. Furious Flame which at max level does +120% Fire damage, and +33% on top of that when Fire damage is done to Shields or Armor, and Flame Talisman, which at max level +18% Damage Resistance and +100% Weapon and Skill DMG per stack, up to 5 stacks, every time you inflict Burning. You lose 2 stacks whenever hit by an enemy, though.
Crown Prince, or Cat, is amazing for any sort of elemental damage because he has quite a few ascensions helping every effect, e.g. Pyrodominance: At maximum level, +120% Fire damage, and double damage done by Burning or Combustion Acidominance: At maximum level, +120% Corrosion damage, and any Critical hit (regardless of damage type) will create a smoke grenade with the same effects as normal, though this can only be triggered once per 3s for every enemy. Electrodominance: at max level +120% Lightning damage, and +120% weapon damage done to any enemy under Lightning effect. Elemental rage: at max level, +120% DMG dealt to enemies under any elemental effect.
Any of these ascensions except for Acidominance can be obtained through Ascension Fusion on other characters, however.
This is the more fun part, but also has some bad news
Each instance of damage, e.g. a projectile, can only do up to 15 million damage (with elemental advantage), 7.5 million damage (with elemental DISadvantage), or 10 million damage if it is not elemental damage. Skill damage has a cap of 30 million, or 45 million if at advantage, or 22.5 million if at disadvantage.
Having high projectiles, high RoF, or high multihit helps deal much more damage, some of the most powerful weapons for this being: Porcupine Angelic Aura Wild Hunt Thunderstorm Justice Rainbow
There are scrolls that help increase damage separate of damage bonuses, e.g. Elemental Magazine, Explosive Bullets, and Fish or Bear paw which has a high chance of doubling damage instances. Explosive bullets is often +22.5 million dps in some instances.
Fish or Bear Paw does -50% damage twice, (-20% when enhanced), but this reduction is done BEFORE damage cap, so it is essentially double damage if you do 30 million per projectile before cap, although the chance to hit twice is reduced for each projectile you have. The "before cap" is also applied to Elemental Magazine, so elemental magazine is often an extra projectile in effect
A lot of scrolls increase Weapon or Skill damage additively, some of the most powerful being: Dual Edge Extravagant Consumption Elite Assassin
There are some ascensions, inscriptions and scrolls which increase damage multiplicatively, which is extremely powerful, these being listed at the wiki at https://gunfirereborn.fandom.com/wiki/Damage_Modifiers (thanks wiki, most of the info here is sourced from there)
There are a few ways to defend against oneshot attacks on the player, the most powerful being Spirit Feline, which only lets damage take up to 14% of your health, although you take double damage. Another way is Damage Resistance, which maxes out at 90%, making you take only 10% damage, however bonuses like Painkiller Lightning stack on top, including: Deflection Shield Misfortune Aura (Spiritual Link) Painkiller Lightning Enhanced Mid-Air strike If all these bonuses are combined, and you have 90% damage resistance, you only take 0.00675% of all damage, which is, incredibly, stupidly, unfathomably fucking BROKEN although obtaining all the bonuses is pretty late.