Video explaining concepts below (and much more):

When speedrunning there are generally few most important tricks:

• horizontal walking (avoid non-melee attacks)
• bottom kiting (squeeze thru stunlocking enemies or just fight them without taking damage)
• 4tile (trick enemy archers into standing still around the corner)
• telekinesis (skip most of the Diablo floor)
• map reading (cathedral, catacombs, caves, hell)
• enemy stun potential and surround potential
• duplicated maps (especially hell)
• mana shield trick (skip taking stunlocks – wasn't used there)
• phasing (evacuate in random direction – wasn't used there)

Horizontal walking While you keep on walking in W or E direction, you cannot be hit by a single non-melee attack, be it arrow, fireball, fire wall. This can be used to bait goat archers closer, maybe buy some times when lava lords or acid beasts throw projectiles at you and creep closer at the same time.

Bottom kiting While you're walking SW, S or SE, when you start moving, the game thinks you're already in the destination tile. When you're getting stunned, you get "teleported" to destination tile. This way you can squeeze by many enemies just by walking downwards and taking a lot of damage (which can heal without worries). As for killing enemies - if they swing and you go SW/S/SE, they'll be out of reach, it's best seen vs the uglies on dungeon level 4. It's a common strategy to kill much stronger enemies.

4tile Classical 4tile means you just get around the corner and walk 4 tiles away. Enemies will be stuck just around this corner thinking they can shoot you, but they can't. This works vs tier1 and tier2 enemies. Higher tier enemies require different tactics.

telekinesis Using telekinesis to pull one lever inside the 3rd Diablo room (the left one). You can skip clearing the 1st (top) and 2nd (right) rooms. Furthermore, you don't have to pull the other lever in 3rd room, just use town portal to go and come back, and the Diablo room will be open for you.

map reading This is overall complicated, but in a nutshell:

• cathedral maps are divided into 9 parts, let's call it NW,N,NE,W,Middle,E,SW,S,SE, both exit and start can spawn only in 4 bottom tiles: Middle, SW, S, SE. No need to explore the top parts of cathedral
• catacombs are very random, but staircases show mostly on big rooms having many inside rooms (like libraries/shrines). Also these big rooms have higher chance of spawning on the very top positions (W,NW,N,NE,E)
• caves have the highest chance of spawning exit locations on the very top left boundary wall (W,NW,N). Next level passages are facing NW->SE direction, up to town faces NE->SW direction
• hell has a symmetrical map, it's divided on 4 (almost) equal quarters, so if you get to know one quarter of the map, you know all about the other quarters

enemy stun potential and surround potential What's most important in Diablo speedruns is not getting stunlocked. In order to do so, you always want to secure bottom kiting "escape route". Some enemies just coil around you and limit your escape, some are circling around, some just maintan safe distance. It's important to know which monsters limit your escape routes, how big are the escape routes (catacombs is crowded because of many doorways, caves have long corridors, hell is spacious). Some more important monsters:

• balrogs looks scary, but they prefer to inferno you, so they circle around much, they are easy to run past
• witches avoid you, but have strong projectiles, it's good to have some magic resist vs them
• hidden class can pop up in the most unwanted time, especially illusion weavers, be very careful if they are on the map
• gloom and horned demons have charge ability and they are very eager to close your escape routes
• uglies are slow, but very methodical in limiting your escape routes
• all bats are slow and lazy, but it makes them occupy 2 tiles (source and destination movement tiles) a lot, so they make it difficult to run past them
• magma demons and lightning enemies circle a lot, but their melee skills are very high, be very careful with deciding whether you go for it or play slowly

duplicated maps Some levels are duplicated, you get the exact map layout, exact staircase location. It's mostly seen in hell levels, but can happen everywhere (it's even possible to get 4 idential maps in a row - only in caves, never got more than 3 in my experience).

mana shield trick Here's some math: if you have less HP than your character level, you can't get stunned. If you have mana shield on while having low HP, you can just run thru the levels without getting stunlocked at all. It's a common technique to grab book of mana shield, damage yourself with fire wall and just go for it.

phasing Phasing is a spell that allows you to teleport in one of 4 directions: N,S,W,E. It teleports you to the tiles on the edge of your visibility (and to tiles adjacent to them, making it 4*9=36 possibilities). When you're cornered, it's good to attract monsters close to you and just phase away so you get past them.