Micolash is perhaps one of the easier boss in Bloodborne, if you come prepared that is. The fight considers of two phases, the first is you hunting him down and him running from you. Once you do manage to track him down, he will do several attacks; a magic based tentacle attack that is a short ranged and a quick 3-4 hit combo. Both attacks can and will take a considerable amount of damage, so be sure to dodge both.
Once you get him down to 50% he will disappear, forcing you to find and follow him again. The second phase is similar to the first however this time he is a bit faster and now does a magic AOE that can one-shot you if you don't dodge.
Still having a few items, such as the Lead Elixir can help you absorb some damage on the 2nd phase, as does have some Fire Paper as he seems to be weak again fire. If you do use the Lead Elixir, be aware that while it does let you get a few hits in and take more damage, it also slows you down, about 45%.