I've seen quite a few rules for standalone / co-op play or Doom but I was never entirely happy with them, so I wrote my own. I've play tested them over a few hours with a mate and so far we haven't hit any major bumps. If you're interested in such things, any feedback would be appreciated.
Note : I used some of the Co-Op Rules from Joseph Laclair's guide so thanks to him for starting me off.
1 Choose a scenario and lay out the board but without items / enemies but with other objects (blocking obstacles, etc).
2 Set the items / enemies / marines for first area.
3 Remove these speciality Marine cards : Careful, Efficient Killer, Officer, Prepared, Survival Ops, Tech Op, Tough.
4 Determine whether you are going to use the Basic or Expansion Invader deck.
5 Determine whether the Basic or Expansion Invaders will be used or whether each Invader will be determined randomly as it is placed.
6 Marines are set up as below.
7 Create two decks (a Brute deck and a Sneak deck) as described below. Remember that although both decks are created, only one set of cards (either the Basic or Expansion Invader cards) will be used to create the decks.
Basic : All Spawn cards, Charge, Rage, Smash, Surprise Attack.
Expansion : All Spawn cards, Charge, Crushing Blow, Rage, Smash, Soul Fire, Surprise Attack, Tick Blast, Wraith Jump.
Basic: Ambush, Dark Energies, Darkness, Dodge, Jump The Gun, Sealed In, Shaky Footing.
Expansion : Darkness, Dark Summons, Descent Into Madness, Dodge, Earthquake.
Use Well Trained (see Expansion rules).
The marines win if all Marines complete the level before the Invader player can score the required number of Frags (see below).
Notes - when the required number of Frags are achieved, the game does not end. Instead increase the number of Brute cards drawn each Turn by 1 for each Frag over the Required Frag figure. Using this rule, games do not suddenly end due to Marine failure, rather they rapidly become more difficult.
1 Marine Action and Resolution.
2 Draw 1 card from Brute Deck and resolve if possible. Discard.
3 Invader Action and Resolution.
4 Draw 1 card from Sneak Deck and resolve if possible. Discard.
If an invader card affects an invader, choose the most powerful invader from all suitable ones.
If an Invader card affects a Marine, apply it to the first, relevant, acting Marine or randomly.
Invaders always spawn as close to the Marines as possible - if there is more than one suitable location, determine randomly. Invaders must not be spawned in areas where they are two big to move if another larger area is available. Invaders will try to spawn in front of the Marines where possible (to inhibit the effectiveness of sprinting through the level).
Most powerful Invader units activate before weaker units (this order is shown below).
Determine if there are any targets within Attack Range of the Invader (Attack Range is equal to the Invader Movement + OTD).
If there are no targets within Attack Range, move the Invader towards the closest target (this is determined each Turn until the target is within Attack Range).
If there is more than 1 target within OTD, then the Invader will select a target randomly, regardless of range.
If the Invader has an area effect attack, then position so that the attack will affect as many targets as possible.
Invaders with area effect attacks will try to harm as many Marines in a single attack as possible, regardless of harming other Invaders. Not hurting other Invaders is of secondary concern.
Invaders with the Watchful ability will position themselves adjacent to as many Marines as possible. Blocking passage for Marines is of secondary concern.
The OTD is based upon each dice having an OTD value (yellow = 3, Blue 0, Green 2 and Red 1). This value is, roughly, the most likely range result that the die will generate. Add all of the OTD dice values up for the Invader’s attack dice and add 1 for each point of Accuracy.