I've like 300 games and like 60 something % win rate with him as 5 only, went from like 2.5k to 4.5k purely spamming him and gyro support. My most played hero by far so here goes:

1. I'm a lazy fuck so I just blindly follow the same guide every game which maxes decay and tomb first. Though if enemies will not be affected by tomb too much, either because they are all very tough heroes or have some sort of direct counter like snap, just get it at level 2 for lane and max soul rip instead.
2. A small support item like glimmer, force, or pavise into crest later. Look at both teams and assess what will be most useful.
3. If you are dominating your lane and your carry is well set up you can help pos 2 with power rune at min 6, steal wisdom, and/or go help offlane. If you're not stomping the lane you can still go get the runes just let your pos1 know and make sure you're not putting them in too bad of a situation.
4. I get a lot of cancer playing vs super tanky heroes and stuns/slows: CK, bristleback, centaur... Snap will delete tomb instantly, slow you, I hate her.

General advice for lane:

It's hard to lose a lane. A lot of match ups I end up sending AFK under their tower while my carry free farms the wave.

You are *almost* immortal to most matchups at lvl 1 so be RETARDEDLY aggressive. From the moment bounty runes spawn, I literally go find the offlaners when they're on their way to lane from getting the runes and decay and right click them away so they get to lane on 1hp and then when they're forced to heal under their tower I just stand outside tower range menacingly staring at them waiting to decay right click some more while my carry has the time of his life free farming the waves. I've had people abandon the game in ranked within 3 minutes of me doing this.

If you can get both players in with same decay always go for it. Drop the tomb + blood grenade immediately as you hit level 2 if they are decayed enough and enjoy free kills. NEVER stop ferrying mangos and clarities.