I have a friend with a Malthael level 200+, he got from silver to masters in 2 seasons. What he played was:
Level 1 - Fear the reaper almost always unless you want to double soak and macro a lot, then take On a pale horse.
Level 4 - Always die alone. Always. This is what makes you win any 1 v 1in the offlane and it is also a really good damage in teamfights.
Level 7 - Cold Hand unless you want to deny heals with touch of death, like palm, ancestral, Alex dragon, then go touch of death.
Level 10 - Usually Last Rites. Tourmented Souls only if you want to combo it with touch of death to deny heals against Alex dragon (seriously it is really good against her) or when the enemy team has too many ways to shutdown your last rites. Nanoboost Tourmented Souls is also really strong because of the CDR, especially with the heals on level 20 from nano. This is also really really strong with Soul Collector at 16.
Level 13 - Here you want Inevitable End almost all the time, it’s a really good talent. 2 seconds of unstoppable on a 20 sec cooldown? Really strong. If you take Tourmented Souls, this is a really good talent to combo with it, because you lose your marks and instantly get them back. Shroud of Wisdom if they have something like pyroblast or other huge spell damage ability. Don’t take it just because the enemy has a Kaelthas or a Ktz, it’s much better to just use your unstoppable against their combos.
Level 16 - Always Soul Collector. This give you nice range on your Q, so you are a little bit safer and the cdr is also really strong.
Level 20 - You want No one can stop death almost all the time. This talent is SO strong. If you die but you get a kill post level 20, you can come back and force a 5 v 4. This is really strong, seriously.
Tips: Malthael is a solo laner and you want him in the solo lane. He is really good when it comes to double soaking or winning the point as well after level 4.
In the solo lane, try not to spam E too much or you will run out of mana really fast. Use your AAs instead.
He can take camps alone really early in the game, so use that in your favour. Just make sure you aren’t losing exp.
In fights, poke with your E + Q and only really engage when your tank engages. Watch enemy cooldowns and be ready to use your last rites. Keep in mind that you get a stack even if the enemy is killed before the last rites kills him, so just use it whenever you can.
Don’t focus too much on “I only draft malthael when they have huge hp frontline”. Malthael is a strong solo laner and has really strong wave clear + merc clear. He only really suffers when the enemy team has too much ranged, but if you have a good healer and a good main tank, you are good to go. Last rites is a really good tool to secure a kill, making you win the fight.
Try to always use your W to dodge skillshots, bodyblock someone or to escape.
If I remember more things I’ll share.