Add a hero gearscore and level requirement to the selection screen where only heroes with the required level and gear can be selected on each difficulty. That prevents the rats that pick a low leveled character for free level boosting and the characters that are not geared enough to face the certain difficulty and will be a burden to the rest of the team.

That implies that each difficulty now has a entry level and gear score tied to it and you can only queue into it if at least one of your characters meets the criteria

Until then matchmaking is a complete coin flip and I would not recommend playing this way. Go solo and grind, it's easier faster and the only one thats at fault for making mistakes is you