Don't attack Janna near the fountain!!!
So, we were attacking the enemy Nexus. This was towards the end of the game. Both of their Nexus turrets were down, and our master Yi decides to alpha strike on Janna (near the fountain). She ults and send Yi into her fountain. After this we lose the fight which costs us the game. Moral of the story, "NEVER attack a Janna near her fountain". You WILL die.
ult is a game changer. Hell, all her abilities can turn a fight in your favor with hardly any effort.