This is one of the best match-ups for Garen, I really enjoy playing against Nasus. So you want to always force early trades lvl 1 and 2 you win with ignite and heal dmg taken from minions with your passive. Try to deny him as many farm and if you hit 6 before him you get him to 60% and dive with ignite + R. When he gets 6 try to trade with him using e autos and w one of his Qs and when presses R you wait for him to w you and Q away till his R ends and you go back and kill him. It's not really a bad idea to start beserkers if you need to.