The topic fully explains it. Describe the best way you would end a team fight with a pentakill. I shall provide an example that doesn't have anything to do with League.
Ok, picture this;
You are playing a MOBA, and you spectate a battle of the team about to play yours. One guy pulls the entire enemy team into one location, then you see 2 others come in and isolate them from the map and begin dealing massive damage to them while keeping them localized. THEN you see the other 2 members release a giant beast and massive storm clouds each dealing massive damage to the enemy group. If any survived the chaos, you would see them obliterated by lightning bolts, finishing them off, what do you do?
Now apply the same concept to League. Oh, and btw, every move mentioned in that paragraph is an Ultimate Attack