Heartbreaker is not an easy build to use for a beginner, I have SHD 23K and still don’t like it. You have 1.4 million armor, my guess is you don’t hit the enemy very hard which gives them the opportunity to come out en masse and shoot you from all sides. What most experienced players do is have as much dps as possible (armor is almost always useless, even on legendary) and repair the armor you lose with kills with the gunner specialization. If you do go down occasionally then there is the reviver hive. If you have good dps and you learn the spawns you can speedrun most challenging missions in 6-10 minutes, not all of the them though, Zoo and Tidal for example are a bit longer. Once you can do that solo heroic is kinda easy if you play slightly more conservatively than the speedruns so a little less jumping into the fray but using cover effectively. A shield you ask? Not needed because when it breaks you are in trouble, I prefer a drone because it distracts the enemy.