Advice on DS2 after DS3

ADP (and to a lesser extent, ATT) increases Agility, which affects the number of i-frames you have on rolls and backsteps. To get the same amount of i-frames on rolls that you had in DS3 you would need 105 Agility.

Pretty much everything is viable in DS2 for PvE and PvP. If you want Dex/Pyro, you go right ahead and do that. The scaling on most weapons isn't terribly important, I would only go to the soft cap of 40 STR/DEX on weapons with an S in scaling. VGR and END are important for every character, get them to at least 20. Leveling up ATT will let you equip more spells and increases casting speed, both important for Pyromancies since they tend to have fewer casts and slower casting time than other schools of magic. VIT is only important if you're going to wear heavy armor, which is really only important for builds using heavy weapons... If you keep VIT low, there's a ring you can get in one of the DLCs, Flynn's Ring, which increases your physical damage by quite a bit.

Any Curved Sword, Thrusting Sword, or Katana will be fantastic.

Some useful pyromancies are Lingering Flame, Combustion (not so necessary if you have a melee weapon), Flame Swathe (boss killer, basically the same as Profaned Flame), Firestorm...

Make sure to upgrade your weapon and Pyromancy Flame, this is where most of your damage comes from.

/r/DarkSouls2 Thread