Tahm Kench as viewed in G2 vs ROC

So I was watching the EU LCS today and noticed that twice during the earlier half of the game that ROC(at the time of the fight the winning team) had 4-5 members focusing damage on Tahm Kench, and it still took almost 10 seconds to take the guy out.

I was at first like "hmm maybe he is a bit to tanky but not sure, besides everyone knows never to focus the tanks in the team fight so thats ROCCATS mistake."

Then I noticed something, Tahm was running Thunderlords Decree, and I watched him take about 2/5 of Lucians health with a single Q. He was dealing serious damage, not quite ADC or APC level but really more on par with what I would expect from an Olaf or Darius similar character. And that got me thinking.

If I am not behind or underpowered during the teamfight, and I cant take the 10ish seconds it takes as a full team to chunk down a tank, as the other threats will free hit me, but I also cant ignore the tank that can swiftly kill carries on my team single handedly...how do I approach the teamfight?