On last boss when you get banished to the catacombs, never backtrack. Take the "longer" route going kind of back first so you run all the doors from left to right or right to left and don't have to retrace your steps. Don't give the hoarvor more chances to shred you than they already have.

If you do run into hoarvor, don't bother fighting them, cc them and roll dodge through them.

When you're about to be back from the catacombs, check where the other 2 are and if they're behind, wait for them if possible. If hoarvor catch up you'll have to jump, but you can usually wait some. Don't stand on the actual ledge too or she will teleport you down, stay inside.
If you're tank&healer you can jump down without too much issue but if you're 2 squishy dps and your tank with healer are barely halfway through their maze, you are going to have a bad time with Veli down there on your own.

Velidreth will spawn the resource draining spores based on the highest resource of the person whose ultimate she gobbles up(and she'll gobble up the person with highest ultimate). So if you make sure your tank's highest resource is health, have him slot his costliest ultimate and never use it, you'll only ever have health spores which you can generally heal through.
It's also a lot easier to avoid her spores by the wall rather than in the center. Some wall is better for this than others but I'm too topographically challenged to remember which Personally I like having her by the wall opposite to the statue so that orb of spite has longer to travel to us. Maybe it's placebo effect but I feel it's easier to avoid this way.

CoS is pretty easy up to the last boss imo, first 3 bosses are more or less just "don't stand in red", Dranos isn't so bad either.