........ANYWAY here's a few tips.
Try not to rely on summons for bosses if you really don't need them, and if you do summon... please try to fight alongside them as much as you can.
I co-op all the time and nothing is more frustrating to me than when my host makes me play the game for them.
That's all up to you and I won't bash you if you do... but just think of the summons! Half the fun of co-op is fighting a hard-fought battle with your host.

And there's a way of knowing the difference between a mimic and a normal chest without even hitting them.
I'll let you figure that out though.

Also, Ornstein and Smough. They took me 2 days of straight attempts (probably totaling over 20-30 attempts) on my base vitality no shield no upgraded flasks/kindled bonfire character.
I basically have them mastered at this point ._.

And congratulations on beating them by the way ^^
That Black Knight Sword could last you till the end of the game if you don't want to farm for upgrade materials. It's a great weapon.
I'd say you're halfway done, but the second half is a LOT shorter considering everything you don't need to do now.