To tell the long story short - we spent around 15 hours in 3 days trying to beat Crota. Then while looking for tactics on reddit sub i found out that every successful kill of Crota have one thing in common. Crota was super passive, shooting people from distance and rarely hitting anything with a sword. On the other hand in our experience he almost never use projectiles and often get up instantly. Also we have alot of knights go thru glass in cristal room and Crota often visited us there. Sometimes he instantly jumped on guy with the sword (usually he shoot in that case) even tho he has no shield and was under heavy fire from 4 ghorns. So after we understand that something is wrong, we start to change host until Crota and knights start to behave normally it took us only around of 10 tries to beat him. TLDR If Crota is rarely using projectiles, chasing swordsman to crystal room and alot of knights go thru glass at the start of encounter when you need to change host until they start to behave normally.