The seven-time champions take a 2-0 lead to the Camp Nou, but the 35-year-old said that Blaugrana will likely control the game on their home patch.
"We have so much to lose and so much to gain," Ambrosini told reporters. "Barcelona will take hold of this game, but our main weapon is the counter-attack.
"We have the grit between our teeth and are ready for any situation."
Barca knocked Milan out of last year's competition with a 3-1 aggregate win.