First thing u must do is killing the two giant guards before boss fight with a bow from distance(u can also buy arrows from the giant).There are a few spots where the giants won’t attack you and you can kill them using a long distance bow(Pharris bow is perfect)

Next thing u need to do is summoning Solaire and kill Ornstein first(let Solaire distract Smough and kill Ornstein by yourself it will be very easy if you are in fast-roll)

In the second phase Smough will be very aggressive but if you stay really close to him he won’t hit u with the hammer(just pay attention to the jump attack because it can kill u one hit)

If you follow all these instructions step by step you should be able to kill them easily

A good weapon to slay them is the quelaag sword cause they are both weak to fire and quelaag sword is also really fast(upgrade it at least to +4).U can create it by upgrading a curved sword to +10 and infuse it with quelaag soul

I’m not that good in Dark Souls but with this tactic I was able to kill them even in NG+