If you go hail of blades I recommend waiting in the bush and engaging as early as possible. I'm pretty sure you can win all trades except for maybe Darius but he'll probably get scared and run away. (cuz you know low elo)

As for dark harvest... Try and get off a few early qs so you can poke him down a bit early. The key here is hitting your q on a single target and getting off that damage. Once you killed him 2 times you can push and soak up the pressure of the jungle. Ulting either the Jungler or the laner, killing them and then killing the jungler /top laner.

I feel like it's best to just soak up the pressure at top and split push till late. As long as you go 1 for 1 (preferably killing the jungler if your top laner is already behind). Of course, it's always best to just don't die and try to create as much pressure as you can. You can also tp bot if you see an opening (if the jungler is ganking bot) and ulting the most important player in the team.

I think late game is the most fun part of morde. YOU COME IN YOU SMASH! Try focusing the squishes or keeping engage champions of your carries with e (just remember you have a long cd so use it wisely).
Most cases if your team is behind and you can 1v1 anyone just ult the person your team can't fight the most. Or someone who has good engage so they have an unfavorable 4v4. Make sure you're able to fight them or else it's pretty much useless to ult them.
AS long as you kill at least 2 of their most important members you'll win the game.