For Logarius, just get like two hits in during the first stage, and if you see him do some scythe back dash then you want to dash to your left or you'll get him by the explosion. Once he reaches his second stage and summons those annoying rain of swords, he'll basically resort to melee which you can just parry him to death. If the raining swords are giving you too much trouble, you can always hit or shoot the sword on the ground he placed in order to stop them.