You would only use ruin II spell when you are forced to move around do to fight mechanics or when the boss is casting something that could kill you and you don't want to get punished by it. Fester spell on the other hand is you bread and butter spell and is the root and source or your every increasing DOT power. In order to see the full effect of fester, you will need to first cast Bio II, Miasma, and Bio I or Miasma, Bio II, and Bio in those orders. Bio is instant cast so you will always want to cast that last. Then you would use fester spell after you have applied those three particular DOTs. Ruin II spell is instant so if you ever need to move or dodge something then that is when you use ruin II. Otherwise, always spam ruin I. Fester spell increases the potency at which the DOTs tick for making them deal more dmg. You can try this on a dummy and see the results or try this on an enemy