Which classes does the guide need?

Firebug and Berserker at level 25, and Gunslinger at level 25 if you own the Glock 18C and Rhino DLC weapons. The Berserker is used only in Objective mode to fetch and deliver parts.

How many waves should I use?

The guide sets 4 waves, since more waves just make death more likely.

At the ZEDs' feet or just in front of them, even against bosses, Fleshpounds and Scrakes: ground fire and shotgun shrapnel do more damage than direct hits.

Perks & Classes

KF2 Solo Hell on Earth on All Maps: Firebug and Gunslinger Build

A first-time guide by a player who attempts every Hell on Earth map alone. It assumes you already know how to move, parry and where to camp, and recommends practicing the map on lower difficulties first, especially Suicidal. The author notes his English is imperfect.

Preparation

Have Firebug and Berserker at level 25. Add Gunslinger only if you own the DLC Glock 18C and Rhino. Berserker is only used in Objective mode for carrying parts, so there is no Berserker build.

Firebug

Dragonsbreath shotgun purchase card Dragonsbreath, the first purchase.

Second Firebug weapon from the guide The second Firebug weapon (Dragonsblaze).

The key trick: do not shoot these shotguns at faces or chests. Shoot at the feet or a little in front of them. Fire spread is deadlier than bullets, and this works even on bosses, Fleshpounds and Scrakes. The only targets to aim at directly are Husks and EDARs, and only if you can hit their weak points at range; otherwise shoot the feet and let the flames work.

Firebug skills with the chosen ones circled in red Firebug skills: Bring the Heat, Ground Fire, ZED Shrapnel, Firestorm and ZED TIME - Pyromaniac are circled.

Gunslinger

Glock 18c purchase card Glock 18c (DLC), used as a single pistol for clearing weak and medium ZEDs.

Rhino purchase card The Rhino, the boss and heavy-ZED killer.

The single Glock 18c has a very high rate of fire and fast reload and melts heads. The dual Rhinos (and the single version) are strong solo: like the Firebug shotguns, shoot near the feet and let all the shrapnel hit, since shrapnel does more damage than bullets. The skill setup includes Bonebreaker, because the Rhino's potential is shrapnel on the legs rather than headshots; the yellow-outline skill is your choice.

Purchase order on Hell on Earth

Set 4 waves. Never buy armor, grenades or ammo for the base pistol early; it wastes money.

End of wave 1: buy the Dragonsbreath with full ammo and sell the Caulk n' Burn. No grenades or armor yet.
End of wave 2: choose Firebug or Gunslinger, since there is no turning back. Firebug: buy the Dragonsblaze if you can, refill Dragonsblaze and Dragonsbreath, buy armor. Gunslinger: sell the Dragonsbreath, buy the Rhino and Glock 18c (the trick of dropping a Rhino and buying another gives extra ammo), and spend the rest on Glock ammo.
End of wave 3: fill ammo to maximum, then armor, then upgrade weapons with what is left.
End of wave 4 (before the boss): buy all the ammo, upgrade main weapons fully, buy armor and, with any leftovers, grenades and base pistol ammo.

Upgrade priority: first the Dragonsblaze and the Rhino (your boss and tough-ZED killers), then the Glock 18c and Dragonsbreath if you are rich. Always prioritize buying the recommended weapons, armor and ammo before upgrades.

Bosses

Patriarch: dodge his rockets and minigun, keep your distance, and bring him to about half health. Do not go much lower, or he runs and heals. Then reload and unload shrapnel or fire until he does not stop, since ground fire and shrapnel deal the most damage.
Matriarch: tougher. Do not get close, avoid being grabbed, and have cover from her laser. Firebug: shoot at her feet and let ground fire kill her. Gunslinger: with the Rhinos, shoot shrapnel at the arm with the laser beam, the only part not fully armored.
Others: keep your distance and shoot with shrapnel or fire until they die.

Objective mode

Change the strategy slightly. Keep the Caulk n' Burn a bit longer to save shotgun ammo in the early waves until you can buy the Dragonsblaze, or switch to Gunslinger and trade the shotgun and Caulk n' Burn for the Glock 18c and Rhinos, then follow the standard procedure. Some Objective maps are hard because of ammo shortages in the first two rounds (the author names Nuked and Zed Landing): buy cheap weapons such as the Medic Pistol, a single 1858 revolver, or fill the Caulk n' Burn and the base pistol to the maximum, but always refill the Dragonsbreath first. On the other maps use the Caulk n' Burn until it runs out, then sell it or keep it until you find ammo boxes, until replaced by the 4-barrel incendiary shotgun, or by the Rhinos and Glock 18c.

Last notes

Some maps are harder than others, and a team is better if you can get one. This build and purchase order only make a solo run easier; it does not guarantee victory. If you are not skilled yet, practice on Suicidal for experience.

FAQ

Which classes does the guide need?: Firebug and Berserker at level 25, and Gunslinger at level 25 if you own the Glock 18C and Rhino DLC weapons. The Berserker is used only in Objective mode to fetch and deliver parts.
How many waves should I use?: The guide sets 4 waves, since more waves just make death more likely.
Where do you aim?: At the ZEDs' feet or just in front of them, even against bosses, Fleshpounds and Scrakes: ground fire and shotgun shrapnel do more damage than direct hits.