Napoleon will be working on unlocking a hidden passage out of the palace. Arno, for 2 minutes, needs to hold off and defeat the incoming foes. You are not starting off against anything too big or challenging. Begin with an aggressive approach against the first two opponents. You will have the better part of a minute to deal with them before the next set of foes arrive.
The second round is notably more difficult. You have 3 opponents at level 3 difficulty. They will work together more given the chance. A quick smoke bomb can be used to stun them and then take them down.
The third round comes in just after you defeat the second. Much the same in terms of numbers. The change is there are 2 Brutes added into the assault. Just wait for the timer to run down after that fight.