This build heavily rely on your ability to get some assists or kills to stack up your soulstealer. For that to happen I have chosen the Mercury boots to secure my survival early game. But if the opposing team don't have a lot of stuns/snares pick Sorcerer's Shoes for more dam. With that said you should be able to support your team right behind the lines of action and make sure the fight turns out in your teams favour. If you get as far as Lich bane, and you have a dissent amount of stacks on your soulstealer (at least 10) you should be able to give your opponents a chock. Because when you get that Lich bane, you will step forward and dominate the map!!! Lich bane and Sonas passive is a really nasty combo, plus ofc you will have your ultimate and hymn of valour to easily empty enemy health bars. If Crescendo is cast right is can devastate the opposing team and the group fight is won in no time :)
GL out there