For me it is an abomination known as "Hand Simulator" and it was nearly the death of me
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Borderlands TPS: facing Shadow Trap on TVHM (didn't play TPS long enough to get to UVHM). Still my personal most frustrating boss I've ever faced. I only managed to beat it 4 hours later after constantly bouncing up on the bounce pads so it was harder for it to hit me. Either that or Destiny PvP.