Pain elementals can't do shit if you get up in their grill.

Bait demons and revanents by getting within melee distance and quickly backing off repeatedly. They can't move when they're attacking

Blue Armor has a better protective value than Green, therefore it's advised to not pick up a Green Armor to replace a Blue Armor until you're way down to like, 50 or so armor at the lowest

an Armor Bonus adds a point to whatever armor you're wearing, if you've picked up a Blue Armor, it will restore (up to 200) for that

if you haven't picked up any armor or it's depleted, an Armor Bonus will give you 1pt of Green Armor

Berserk multiplies the damage of your fist by 10, which gives your punches roughly the damage of a full pattern SSG shot

Berserk lasts the entire level, in spite of the red fading away

Berserk also restores up to 100 health

'tapping' with the Chaingun: the first two shots with it are always accurate, but the pattern opens up if you're holding down fire, thus tapping fire repeatedly is better for long range, and longer salvos make more sense at close range

Super Shotgun gives you more damage per shell, but the shorter useful range may not seldom mean that firing single Shells with the normal Shotgun is more economical

monster melee attacks do not check for vertical range, be wary of Cacos biting you from way up high, or Pinkies biting you from way down low

splash damage has infinite vertical range

Cyberdemon and Spider Mastermind do not take splash damage

Revenant missiles and Mancubus fireballs do not have splash damage

Mancubus fireballs can at times clip through solid walls, so don't ever stand perfectly still if taking cover from a volley of their fire if you can help it

Arch-Viles have a hair trigger and can shift their targets on a dime, if one is currently attacking another monster, don't shoot him yet, because then the attack he meant for another monster will likely snap towards you immediately so it stalls them.

Run forward while strafing for a slight speed boost.

To defeat the Cyberdemon, shoot at it until it dies.