He is pretty underrated in teamfights IMO. He brings a very nice long lasting team ad buff, a suppress, and is very hard to kill. Also will most likely be moving very fast because someone will trigger his E.
General tips is wait for someone else to engage if possible, and surpress ap/ac with R. Don't initiate with it because you WILL be CC'd out of it. Don't initiate with Q, save it for after you get hit to heal back. Between a bruiser build, his natural steal, botrk passive, spirit visage passive, and his Q and botrk active he can be very, very hard to put down unless focused.
As others have said spam Q during suppressed ap/ad to hit them immediately after and get some health back (you will most likely take some damage from the other team even if they can't cc you off their squishy. Don't be afraid to use botrk active after too to help finish.
If you can take out just one of their carries it's pretty teamfight changing. --- Strong people are harder to kill than weak people, and more useful in general -Mark Rippetoe