I'd say the whole game is about spacing and timing. The roll poke is king of dks combat, and being right at the edge of your range and attacking them mid roll/attack ensures they can't respond, and only attacking once (twice if you both connect and stun them) ensures they can't toggle out or setup parry you, all other attacks are really just bait to attempt to get them to do something you can punish.
The only thing to watch out for if you're doing this correctly is being predictable, if they guess your attack is coming, especially with a heavy weapon or magic, they're probably going to kill you and you won't be able to stop it.
The reason to learn to BS is to avoid being bs'd, because against a good player most attempts are likely to fail and serious attempts instead of just threatening them are probably going to get you counter bsd or hit in the face with an unlocked attack. It only works at specific spaces and times. If they're moving forward don't attack, if they're hovering just inside your weapons range don't attack, if they're straffing in a circle take a couple steps back as they get close and bs them for being stupid, should keep most attempts from success.
Don't block people using any UGS, the great scyther or any greatsword in 1 hand, it doesn't work, they're going to dead angle.
If you get bs'd, and your opponent stands behind you as you get up, only a couple things can happen, you just have to guess and pray.
1: A kick followed by another bs, Roll bs them to counter or roll any direction but directly away to escape. Can be tough to time, roll ASAP.
2: An attack (typically an r2) or a bs without a kick, you can roll bs or flick the joystick towards them and hit parry (again, tough to time) or eat the attack without pressing anything (it won't be a bs so long as you press nothing) or roll any direction but directly away to escape.
3: A spell. Easily the most dangerous if done correctly, roll bs to punish or straight to a side to avoid most of them, timing changes with the spell, which is the *****, so the common spells used typically are what I'll address. GC you roll towards them unlocked, WOG you roll sideways while locked on, fire tempest you roll bs or run and pray, HCSM you avoid to either side locked or unlocked but you have to wait for it to fire, pursuers you just run from, DB you roll through them like you would GC.
The worst one is great chaos fire ball. You cannot avoid the 300 or so lava damage, and if the orb hits it's another 800, and it's easy to manually aim as you try to escape. I'd try to roll straight back, which exposes you to another bs if they guess correctly but they like to throw it right at their feet to catch people trying to roll bs them.
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