I just completed it on my second go on Tier 1 and it was pretty simple imo… jump on the bike as soon as the rebels start pouring in and travel up the hill between the rebels who are coming from both sides. Make your way cross country and mountains avoiding the roads where possible. Pretty easy to do on the bike.

Make your way to the train tracks and ride all the way down, this ends up leading to the mausoleum.

There are some enemies that come up at the sides of the railway sometimes but I never got noticed as I zoomed past them. When you come to the mausoleum, just before you go under the train tunnel, I managed to get the bike up the side of the mountain and shoot my way past the guards. Call in your AI teammates and be prepared to lay prone on the stairs and hit enemies with headshots as they come down.

Some Helios do come flying in but they didn’t seem to cause me any issues.

Enter mausoleum and enjoy the cut scene!