This guide will explain those not immediately obvious mechanics, everything in this guide will help a lot for attempts at Ultraviolence and beyond.
Warning: this guide contains minor spoilers.
You can do this too: just manually target and attack the door.
Often provides better positioning. With a melee weapon and good awareness, you can break a door and move into a doorway to be adjacent to an enemy on the other side without giving them a turn to attack you first. With a ranged weapon, you can save yourself a turn of moving into cover.
This also works on locked vaults except on some special levels and during the “Secure Vault” level event. Not spending time finding or running to the terminal can save you a bunch of turns on Nightmare, limiting respawns.
Line of Sight and Targeting
Normally, you automatically miss if you try and fire at an enemy outside of your line of sight.
There are some exceptions, however:
1. Shotguns can’t miss. This means you can hit things beyond the edge of vision with them if they have enough maximum range. This won’t do much damage on its own unless you have a shotgun with huge optimal range, but it can allow you to apply status effects to helpless targets at the expense of ammo.
2. Splash damage also can’t miss. This means you can shoot a rocket at a wall and damage whatever happens to be nearby, even if you can’t see it.
3. You can’t miss terrain. This includes explosive barrels.
4. While vision-blocking effects like smoke and toxic gas limit what you can see, you can still fire through them with manual targeting and hit things, assuming you know where the enemies are (and they’d be visible if the cloud wasn’t in the way.
You can’t move diagonally under normal conditions, but you can melee diagonally and will move into the tile if you kill its occupant (unless holding Shift or the equivalent controller button). Combined with manual targeting, you can move diagonally through destructible cover if you have a powerful enough melee weapon equipped.
Why Fiends, Reavers, etc. always end up behind you
These enemies (and more!) can track you by scent, and will roughly follow your path through the level once they pick up your trail. This is also why they can perfectly follow Scouts even in stealth.
Clouds of smoke and toxic gas will break your scent trail and confuse them, as will teleportation. Scent-tracking exalted enemies with the HUNTER trait can still follow you through gasses.