Dude,I honestly thought this was a joke post when I started reading it and saw the title. I can’t say about PvE,but in PvP Miko IS the strongest healer.
for healing beam you just have to think of it more as of an AIM skill,so that you carefully aim the ally you want to heal,second,(at least in PvP) DON’T attack AT ALL unless your team is full or near full hp,rest of the time just focus on healing. For XP use the shroom bomb on enemies(in PvE)to get minion kill assists. As to your ultimate-placement is key. Set it out of sight and in places where allies could take cover. The last,but still important thing is the team working with you- if they run away despite being healed- they draw you into danger and away from others you might have to heal later.
But most importantly,If you want to be successful as Miko you have to focus on the DON’T ATTACK bit. Constantly heal,learn to keep track of your allies and heal,heal,heal.

