Automatically by destroying enemies and clearing maps. Each and every enemy the player faces on a map gives a certain amount of experience and Hell, based on what level the enemies are. The higher level they are, the more Hell they will offer.
A player will not receive any Hell if they destroy an enemy in the Item World, then leave either via Mr. Gency's Exit, or through the portal to the next level.
Through the Bonus Gauge. Depending on the difficulty of the map, the hell bonus can be quite small, or incredibly large. (For example, the Tutorial Map of the first Disgaea could have a hell bonus of around 7 - 12 HL, while the fight against Baal could offer up to 350,000 HL as a bonus.) It is possible for a player to receive multiple Hell bonuses in this way for completing just one map. Upon clearing some maps in Disgaea 2 and its remake, if a HL Bonus is high enough, it will say "Super" instead of the actual massive total.
By selling items and equipment. Usually, a player will receive only half of what an item is truly worth (Specialists, Rarity, and level of the item can all add to this value).
Though only a few times throughout the whole game, a player can receive Hell through storyline play.
In Disgaea, a player receives 300 HL after clearing the Tutorial map in order to heal their characters at the Hospital. Also, a player receives 5000 HL from Hoggmeiser when he joins the team at the end of Chapter 3.
In Disgaea 2, a player receives 300 HL after clearing the Tutorial map in order to heal their characters at the hospital. This only happens on the first play-through, as the tutorial is skipped completely on each additional play-through.
By passing the bill 'Extort Money from Senators' ('Raise Military Funds' in Disgaea 1). However, this is a very useless way to get Hell, as you must often pass the bill by force, since all senators tend to hate it, and the cost of healing your characters afterwords is usually more than what you receive. The least one can receive from this bill is 10 HL.
In Disgaea 2, become a senator and get as much influence as possible. Go to all the meetings you can and take as much as possible from the bribe bag. You can return to HQ after obtaining the items and still keep them. The Prism Rangers give you items that sell for millions.
In Disgaea 2, certain felonies give HL as a reward when a character enters the Dark Court. Specifically, High stat felonies give 50,000HL; Too High stat felonies give 2,000,000HL; and Way Too High stat felonies give 500,000,000HL!
Healing. When talking to the healers in any Disgaea game, a player must pay Hell in order to heal the team. The more damage the player's characters took, the more SP the characters used, and any destroyed characters will all add to the HL cost of healing the team.
Buying items and equipment. By buying better equipment, a player's characters can become stronger, either offensively or defensively, and by buying items, a player can either heal a character, restore some SP, heal a status effect, or even steal items from enemies. Buying items and equipment can also help for bribery at the Dark Assembly.
Buying Music Tracks, Character Colors, and Illegal Tickets. These items will cost a fortune, some of them costing over 10 billion HL.
In one non-plot-related cutscene in Disgaea 1, Etna and Jennifer are seen in an infomercial-style commercial, trying to sell the Super Robot Thursday (and a bonus branch chopping set) for "only 29,990 HL"
In Disgaea 4, Artina reveals that Celestia uses a different type of currency called CL and that the exchange rate is abysmal with 100 HL = 1 CL.
Having Hell as currency may allude to the saying "There will be hell to pay".
Extremely high value items such as a Trapezohedron will only sell for a very low amount. This is due to the item's sell value having such a high number, it overflows and only displays the first digits in the shop.
The Broker Specialist will increase HL depending on the value (i.e. a 300 Broker would result in 300% increase in HL gain).