1) If your tank hero can’t sustain the Boss's normal attack, better to increase your main heroes’ power first.
2) Most major skills may give a warning before being launched, with a red range indicates its effect area. You need to interrupt it or move out of the red range to avoid the damage.
3) Some skills have no warning but also very deadly. However if you are familiar with the Boss skill mechanism, you can observe the Boss action and anticipate it, and then interrupt it or avoid it. That’s the key to defeat the Boss.