A short primer on the mechanics that actually decide fights in Half-Life 2: Deathmatch -- weapon tricks, movement, and the habits that separate new players from experienced ones. None of this needs any addons or config changes beyond a couple of console binds.
Nade Cooking
Use the gravity gun to time your grenades. grenade +attack2, switch to gravity gun, ggun +attack2 to pick it back up, ggun +attack to throw it. Every pickup resets the fuse timer -- you control exactly when it goes off.
Backpack Reload
Guns reload automatically if you switch away and wait a couple of seconds. Fire the crossbow, switch to SMG, switch back -- already reloaded. Only the SMG and AR2 don't backpack-reload.
Shotgun +attack2
Use the shotgun's secondary fire. You want to land as much damage as possible in as little time as possible, and the alt-fire is a big part of that.
Suit Armor
Absorbs 80% of incoming damage (you'll still always take at least 20% as HP damage). Makes you far less likely to be one-shot -- without suit, a crossbow bolt will one-shot you.
SMG Nades
Good for spam and effective against an opponent's suit. Best used as a pre-fire before rounding a corner -- mid-fight it risks your opponent shooting it back in your face.
AR2 Orbs
An instant-kill spam tool. Fire the alt-fire and push into a room or flank around it. Also a good distraction to bait your opponent into drawing their gravity gun mid-fight.
Fastorb
Use the gravity gun to push an AR2 orb at enhanced speed: AR2 +attack2, switch to gravity gun, ggun +attack to punch it forward.
Weapon Combos
Shotgun alone often isn't enough; pair a pre-fire weapon with a finisher. Common combos: SMG nade + shotgun, gravity-nade + SMG, crossbow + shotgun, magnum + AR2.
Headshots do 3x damage
Compared to body hits -- a magnum headshot can do 225 damage and end a fight instantly. Always aim for the head.
Gathering (& spawning)
Know where every item pickup on a map is, and gather efficiently. You should know where to grab a gun, suit or spam item the instant you spawn -- a gravity-nade jump can get you there faster. Also doubles as learning spawn points for spawn frags.
Repositioning
If you get hit and retreat, know where nearby items are so you can quickly stack back up while repositioning instead of running in blind.
bind mwheeldown +jump -- how most players bunny-hop. Spacebar bhop is possible but harder to keep consistent; type the bind above into your console to switch.
Strafe Jump -- the big initial sprint-jump that starts a bhop chain, also used to clear gaps. Hold forward + sprint + strafe, turning your mouse in the direction of the strafe, then release sprint and forward once you're airborne while still holding the strafe key.
Crouch Bhop
After your strafe jump, hold crouch while bunny-hopping. It keeps you silent and conserves sprint.
Sprint Conservation
Running out of sprint mid-fight leaves you vulnerable to attack -- never let it fully drain if you can help it.
cl_showpos 1 -- console command that displays your speed/velocity. Aim to maintain around 400 velocity while moving.
HL2DM University (must read)
Ade's Steam guide
Edge's Steam guide
HL2DM on PCGamingWiki
Guide content adapted from the community-run HL2DM New Player Introduction.