It is not effective, because you cant kill *** with that. Everyone i saw using it (especially together fight knightslayer) struggled to do decent damage in duels. And some of them just got bursted while the cooldown was ticking. But maybe its a console issue...
what i stated is from the PC EU server, where duelling is a very active content and good/best players normally gather at bergama wayshrine. and in the last half year everyone with meridias blessing set normally had no damage at all (also in combination with knightslayer) and the fights just dured several minutes and ended in a draw, because nobody was able to deal serious damage to the enemy. the only setup with which this works is in a werewolf setup, where the werewolf bleed in the is the deciding factor, But in any other form, a combo of meridias and knightslayer havent won a real duel in the last half year there.
meridias together with knightslayer is a bad combo. only one player i know is using it and he cant kill decent palyers with it, because the damage sucks and he also gets smashed in duels despite ignoring enemies half the times thx to meridias.