The problem people seem to have is they allow themselves to get bottlenecked, where the enemy is spread out directing all of their 'fire' into a tight area where all the PCs and NPCs are located. By this level your party should have the means to misty step, invisibility, shadowstep, jump, fly, gaseous form, etc. What you need to do is get some PCs behind the enemy.
In my playthru, I used my Shadow Monk to basically teleport behind the enemy, and Lae'zel to jump behind their front rank. The Harper druid (not using her name to avoid spoilers) wildshapes and jumps behind them as well. Now the enemy needs to divide their attention.
Meanwhile those up front need to keep pushing forward. Spirit Guardian spell, Spiritual Weapon, Moonbeam, anything area of effect can help clear the path.
Then there's explosive arrows, pushing attacks, etc. that can help push the bottleneck forward.
Getting someone up into the rafters with a bow helps too.
