First, compute the mission difficulty:
| Skill Level | Easy | Normal | Hard | Insane | Brutal |
|---|---|---|---|---|---|
| Base Difficulty | 3 | 5 | 7 | 10 | 13 |
If rd_difficulty_tier is active, add 12 times its value to the mission difficulty.
If the campaign has a DifficultyModifier entry for this mission, add its value to the mission difficulty.
Next, add a modifier based on the number of marines present in the mission. If asw_adjust_difficulty_by_number_of_marines is set, the modifiers for marine counts below 4 are applied. If rd_increase_difficulty_by_number_of_marines is set, the modifiers for marine counts above 4 are applied. Most ASBI-based modes have the latter set, and most other modes have the former set.
| Marine Count | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| Modifier | -2 | -2 | -1 | +0 | +1 | +2 | +3 | +4 |
The mission difficulty has a minimum value of 2. If it would be below 2, it is increased to 2.
Alien health is increased or decreased by 20% for each point of mission difficulty above or below 5 (normal).
| MD | 3 | 5 | 7 | 10 | 13 |
|---|---|---|---|---|---|
| Allies | |||||
| Marine* | 80† | 80† | 80† | 80† | 80† |
| Colonist* | 40 | 40 | 40 | 40 | 40 |
| The Swarm | |||||
| Drone | 25 | 40 | 56 | 80 | 104 |
| (Uber) | 300 | 500 | 700 | 1000 | 1300 |
| Ranger | 60 | 101 | 141 | 202 | 262 |
| Shieldbug | 600 | 500 | 1400 | 2000 | 2600 |
| Buzzer | 18 | 30 | 42 | 60 | 78 |
| Boomer | 480 | 800 | 1120 | 1600 | 2080 |
| Parasite | 15 | 25 | 35 | 50 | 65 |
| Harvester | 120 | 200 | 280 | 400 | 520 |
| Xenomite | 6 | 10 | 14 | 20 | 26 |
| Mortarbug | 210 | 350 | 490 | 700 | 910 |
| Mender | 35 | 59 | 82 | 118 | 153 |
| Grub* | 1 | 1 | 1 | 1 | 1 |
| Queen | 2100 | 3500 | 4900 | 7000 | 9100 |
| Zombies | |||||
| Classic | 30 | 50 | 70 | 100 | 130 |
| Fast | 30 | 50 | 70 | 100 | 130 |
| Poison | 105 | 175 | 245 | 350 | 455 |
| Zombine | 60 | 100 | 140 | 200 | 260 |
| Headcrabs | |||||
| Classic | 6 | 10 | 14 | 20 | 26 |
| Fast | 6 | 10 | 14 | 20 | 26 |
| Poison | 21 | 35 | 49 | 70 | 91 |
| Antlions | |||||
| Classic | 14 | 24 | 33 | 48 | 62 |
| Worker | 14 | 24 | 33 | 48 | 62 |
| Guard | 300 | 500 | 700 | 1000 | 1300 |
| Guardian | 300 | 500 | 700 | 1000 | 1300 |
| Combine | |||||
| SMG | 30 | 50 | 70 | 100 | 130 |
| Shotgun | 30 | 50 | 70 | 100 | 130 |
| Elite (AR2) | 42 | 70 | 98 | 140 | 182 |
| Hunter | 126 | 210 | 294 | 420 | 546 |
| Strider | 210 | 350 | 490 | 700 | 910 |
*Some creature types do not have health scaling and use the same max HP regardless of difficulty.
†Marines gain max HP for each point in the Health skill.
Similarly to health, alien damage increases or decreases by 20% for each mission difficulty above or below Normal. In Source Engine, fractional damage carries over and there is no rounding. All attacks deal a minimum of 1 damage before calculating any resistance for the victim.
| Attack | Damage (Normal) |
|---|---|
| The Swarm | |
| Drone (slash) | 15 |
| Drone (blocked while climbing) | 20 |
| Ranger (spit) | 6 |
| Shieldbug (stomp) | 25 |
| Buzzer (sting) | 15 (max 30) |
| Boomer (kick) | 4-6 |
| Boomer (explosion) | 55 (does not scale) |
| Parasite | (infest damage is special) |
| Harvester (skin) | 5 (does not scale) |
| Xenomite (burst) | 15 |
| Mortarbug (skin) | 5 (does not scale) |
| Mortarbug (explosion) | 50 (does not scale) |
| Mender | (does not attack; healing does 4% of max per second) |
| Queen (tentacles) | 1 |
| Queen (slash) | 5 |
| Queen (spit) | 10 |
| Zombies | |
| All (swipe) | 10 |
| Poison (swipe) | 20 |
| Zombine (grenade) | 200 |
| Headcrabs | |
| All (leap) | 5 |
| Poison (poison) | takes marines down to 1 HP; 20 to other targets |
| Antlions | |
| Classic (kick) | 10 |
| Classic (leap) | 10 |
| Worker (kick) | 10 |
| Worker (leap) | 10 |
| Guard/Guardian (shove) | 10 |
| Guard/Guardian (charge) | 20 |
| Guardian (poison) | takes marines down to 12 HP; disabled by default |
| Combine | |
| Soldier (SMG) | 3 (per bullet) |
| Soldier (Shotgun) | 3 (per pellet) |
| Soldier (shove) | 10 |
| Elite (AR2) | 5 (per pulse) |
| Elite (shove) | 15 |
| Hunter (kick) | 20 |
| Hunter (charge, if marine) | 20 |
| Hunter (charge, unless marine) | 250 |
| Level | Experience from Previous | Total Experience | Unlocked Weapons |
|---|---|---|---|
| 1 | N/A | 0 | Starting Weapons* |
| 2 | 1000 | 1000 | L3A Tactical Heavy Armor |
| 3 | 1050 | 2050 | Model 35 Pump-action Shotgun |
| 4 | 1100 | 3150 | X-33 Damage Amplifier |
| 5 | 1150 | 4300 | IAF Tesla Cannon |
| 6 | 1200 | 5500 | Hornet Barrage |
| 7 | 1250 | 6750 | Precision Rail Rifle |
| 8 | 1300 | 8050 | CR-18 Freeze Grenades |
| 9 | 1350 | 9400 | IAF Medical Gun |
| 10 | 1400 | 10800 | Adrenaline |
| 11 | 1450 | 12250 | K80 Personal Defense Weapon |
| 12 | 1500 | 13750 | IAF Tesla Sentry Coil |
| 13 | 1550 | 15300 | M868 Flamer Unit |
| 14 | 1600 | 16900 | v45 Electric Charged Armor |
| 15 | 1650 | 18550 | IAF Freeze Sentry Gun |
| 16 | 1700 | 20250 | M478 Proximity Incendiary Mines |
| 17 | 1750 | 22000 | IAF Minigun |
| 18 | 1800 | 23800 | Flashlight Attachment |
| 19 | 1850 | 25650 | AVK-36 Marksman Rifle |
| 20 | 1900 | 27550 | IAF Power Fist Attachment |
| 21 | 1950 | 29500 | IAF Incendiary Sentry Gun |
| 22 | 2000 | 31500 | FG-01 Hand Grenades |
| 23 | 2050 | 33550 | Chainsaw |
| 24 | 2100 | 35650 | MNV34 Nightvision Goggles |
| 25 | 2150 | 37800 | IAF High Velocity Sentry Cannon |
| 26 | 2200 | 40000 | MTD6 Smart Bomb |
| 27 | 2250 | 42250 | Grenade Launcher |
| 28 | 2300 | 44550 | PS50 Bulldog |
| 29 | 2350 | 46900 | IAF HAS42 Devastator |
| 30 | 2400 | 49300 | 22A4-2 Combat Rifle |
| 31 | 2450 | 51750 | IAF Medical Amplifier Gun |
| 32 | 2500 | 54250 | TG-05 Gas Grenades |
| 33 | 2550 | 56800 | 22A5 Heavy Assault Rifle |
| 34 | 2600 | 59400 | IAF Medical SMG |
* Starting Weapons are:
| Promotion | Name | Experience Requirement Multiplier | Total Experience to Max |
|---|---|---|---|
| 0 | N/A | 1x | 59400 |
| 1 | Titanium Star | 1x | 118800 |
| 2 | Carbide Star | 1x | 178200 |
| 3 | Gallium Cross | 1x | 237600 |
| 4 | Platinum Star | 2x | 356400 |
| 5 | Osmium Star | 4x | 594000 |
| 6 | Iridium Medallion | 6x | 950400 |
| 7 | Golden Medallion | 8x | 1425600 |