Hey clashers, Thanks a lot for your valuable suggestions. Even though, my PD has not yet reached 7/10, my heroes survived for full 3 minutes and did 31 million damage in Boss 3 today. If it was not for your advice, I would have taken PD to 6/10 and then started upgrading another Hero. And wouldn't have got the same results as today.
Earlier my PD was the first one to face Boss, but today, Boss attacked Vlad first and Vlad was able to survive the attack, due to his healing and Druid's healing. And when first round got over, I checked the score. It was 31 million. It was awesome as yesterday I ranked 5th in my Guild in Boss and today I was 1st.
Thanks again.