You should also try gift-dropping. All types of formers plus hell knights and barons love picking up armor, medpacks, and phase devices. So if you think there's some formers behind a door, you can set a medpack or other gift in front of the door, get diagonal to the door, and open it. Any formers will come running for the gift, and you can give them some shotgun to the face when they get in the doorway. You probably shouldn't do quite the same thing against hell knights or barons, because you can't just shotgun them in the face for a one-shot kill. A melee build with a chainsaw and the Berserker trait can handle them, though. You can still use gift-dropping to lure them close to you though.
If you want to corner shoot with a non-shotgun weapon, there's a similar setup:
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B.......Basically, by taking a step back you can see where you're going to be shooting, so this is great for chainguns. If you chainfire against something even as strong as a hell knight or cacodemon at the end of your vision range, you'll probably kill them before they get close enough to shoot you, especially if you have some levels in eagle eye and son of a bitch or trigger happy. You could also place a gift right around the corner there to lure hell knights in. One thing to be very careful about, though, in any corner shooting or gift dropping situation, is making sure enemies aren't going to come from a different angle. For example:
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.....B..You won't have any cover against the enemy coming at you, so you'll probably have to retreat around the pillar and maybe even abandon your gift. (I didn't actually count the squares so this probably doesn't reflect your actual vision range)
And on that note, it's important to know what areas are safe to retreat into. One common trick is to close doors after passing through them, since you can tell by looking at the map whether any enemies have since opened them. You don't have this luxury in maze levels; I recently lost a character on Ultra-Violence when (despite having a tracking computer >_>) either a baron or hell knight came around behind me as I was fighting some imps near the edge of the map. I didn't have any place to retreat to except forward into the imps (and some barrels!). It's not so important on lower difficulties, but phase devices are good for escaping these kinds of situations. Yeah, sometimes they'll put you in worse situations, but if you know you're screwed it can at least give you a chance to survive.
You may think killing 100% of the enemies and clearing all special levels makes you awesome, and it does =), but when you're still learning the ropes, skipping special levels and leaving levels early can be important. Once you find the stairs down, you should try to explore the level and find any supplies and kill some enemies, but be prepared to escape; you should probably clear out the area around the stairs first so that it's safe to retreat. As for special levels, Hell's Arena is a good place to practice corner shooting, so I would say keep doing that level. And if you do clear Hell's Arena I'd avoid the Chained Court because of the Arena Master. The Halls of Carnage is probably the only other special level I'd recommend (in fact, you should do it no matter what).
