The only missions that are problematic are The Renegade, The Archive and The Saboteur, if you play them on advanced with any more than 3 players you will surely fail as the defense points do not scale with more players, I’ve done these three missions on advance with 3 players but never beaten them with 4 or 5.
The Heliophage is stupidly difficult, but it’s supposed to be. Again, play with 2-3 players, if you play with 4-5 the enemies become too strong and spawn so erratically that you’ll be quickly overrun and not complete the first phase in the 23 minute time limit.
It is, ALMOST impossible. I have a friend who completed it solo on advanced with Toby, he helped me in a 3 man, it can be done. Perhaps you need to change your character or your strategy. I like how hard the final boss fight is because it’s so rewarding when you succeed. My best tip would be to isolate Rendain in his final phase under the tunnel on the green islet and focus on nothing but destroying him before the Thrall Brutes reach you, if Thrall Brutes spawn here take him to the large islet where The Gunhulk spawns and fight him atop the stairs. It is frustrating to fail after an hour of grinding but persevere and you will succeed! There are plenty of players who have done it and plenty who still haven’t, find a good team of 2-3 and take Rendain on as a group rather than by yourself then plan your solo attack once you’ve completed it in a team.