Undertow is best for picking off early kills and keeping people from getting away. after your first kill or two go b and pick up either your berserkers or giants belt depending on gold atm. stay in lane for the most part (not top) and only gank if you know YOU will get the kill. going back with any oppurtunity to buy making sure you rush warmogs after you get your boots.

You should be pretty fed with your undertow sniper skills and your boss ganks, your frozen mallet will ensure noone will ever escape your grip paired with undertow and the extra hp just feeds your w. Thornmail can easily be switched with a force of bature if your playing an ap heavy team or even with a phantom dancer if you feel you dont need the defence (i have done it) in team fights you should be nearly unkillable useing your ult for escapes and your undertow to prevent them. Please note your e IS NOT YOUE INITIATE UNLESS YOU ARE AT THE DISAVANTAGE OF PLAYERS (1v2,1v3,2v3) it is used to ensure you pick up the last blow. also. once you are at less than half hp, use your w and feel free to tank a turret or two. trust me. you can

By now you should be completly unstoppable. You should be able to 1v3 most team combination. now is just time to have fun XD kill as you feel and win hen you please, The bloodrazor can be switched with almost nything you feel fit. i like t becasue most 3v3 are being played with high hp chmps so it is usefull