T4 Boss info:
T4 boss cannot be stunned, so the same set of heroes that you use
for T3 boss will not be effective. There are several strategies for
lasting a full 3 minutes against the T4 boss, either with slowdown
(perma-slow), silence (perma-silence), or massive damage debuffers.
The easiest method by far is perma-slow. War-machine has several
extremely useful videos, showing different strategies and team
compositions for heroes, talents, and crests. I'm not going to bother
repeating his information here, so just check it out for yourself: