They were just constantly knocking hopeful balls into the box and Forest had 5/6 players waiting to clear it. And Hojlund was like a baby trying to stand up on a bouncy castle. Maguire won 3 headers when he came on, which was more than Hojlund would have got in 4 games.
When you play against a team set up to defend like that, the worst thing you could do is just keep crossing it in. That's what they want you to do. They are set up for that.
Going back years, when Pep was met with the same predicament, he would have Mahrez or Stirling take it right to the byline, get behind them and cut the ball back for Silva, De Bruyne to get a tap in.
That is how you beat a team set up to defend their box.