She takes a little bit to pick up. I've gotten quite a bit better with her but am by no means a master so take what I say with a grain of salt.
- Build:
You don't always need to build black cleaver like you're doing. I've personally found that if the enemy is squishy and not building armor, that Triforce on her works extremely well. The sheen proc synergizes very well with lunge and the low lunge CD. Your build should always be hydra and then either black cleaver or hydra depending on how squishy they are.
After that it's situational but Spirit Visage and Deadman's work very well for her. If you want more offensive it's always been either Maw, Bloodthirster or Death's Dance.
- Runes & Masteries
Personally I've found that you either go AD quints, AD reds, Armor yellowsand CDR per level blues (6) and MR blues. Replace the AD reds if you know you're going to be building black cleaver with attack speed reds as it will really help when stacking that armor shred.
Masteries is simple it's 21/0/9 taking fervor and the extra healing in the defensive tree
- Early laning
Almost always build a doran first. You can play aggressive against certain match ups. You go Q, W, E at levels 1 2 & 3 then max Q first then E then W taking your ult whenever possible.
You want the vital in front of you which you can reset by moving away from them in lane. The basic lane harass is to Q into a vital. Auto then immediately E to rest your auto which will slow with the crit. After that you can continue to auto if they run or use your W if they fight back.
- Mid Game:
Push push push. You are a split pusher and should be doing just that. Early teamfighting can be very dangerous for you as you are quite squishy. You'll do tons of damage with your first two damage items but you won't have your defensive items yet. Keep pushing and farming CS using your teleport as needed. Your third item should almost always be a defensive item, either SV or Deadman's. There's really no champions that can 1 vs 1 you.
- Late Game:
Grouping is obviously much more important. You'll have the tankyness to survive but not engage first. You want to pop your ultimate for your team and try to get it off which can be hard but you'll get to use it. Split pushing is still an option at this point depending on their team comp and yours so be open to that.