Dodge his Death Ray and within the next 7.5-ish seconds he's basically a free kill unless of course he's ridiculously fed in most duels/teamfights. His Death Ray is his primary poke and damage output.
Also remember that his Q's damage is split into two halves, one is his actual Q which doesn't do all that much damage and only gives him the shield mainly, but the other half is his auto attack which has less range than his Q. So abuse the range difference.
Also remember this his ult's only damage isn't just over time, when he spawns it he does a nice burst but the damage over time isn't that scary, It's why he has such a big spike at level 6 though due to the ult being a huge AoE burst that's fairly hard to miss, but it can mess up teams if they decide to not stay into it.
His early game is pretty weak UNTIL he gets his first passive upgrade by the way, It's because he basically gains like 40% more damage on his E, don't get me wrong he can still kill you before that, but I find it similar to Akali. You farm until level 6 and then kill everybody. It's the same with Viktor except you can hit this power spike sooner or later depending on your farm to 1,000 gold, and you also can be more offensive than an Akali pre-6 would be.