Um…that is your problem (or one of them). Your heroes aren’t strong enough. Level 60 Richard? For Legendary? Not happening.
Lord knows there are many things to legitimately complain to the devs about, this certainly isn’t one of them. The Legendary stage (and definitely the last few levels) is meant to be a challenge for advanced players. I am level 47 and team power over 4000 with many maxed 5*s and I find it a challenging thing to finish.
When I first started playing and developing, I too couldn’t finish the advanced stages of events…that is normal. Get stronger, more skilled in game play and experienced and you will eventually be able to complete the event.
Nothing to see here.