If i attack with empowered ambessa cost 5 to an opponent unit with master yi as legend, does the +2 trigers when I want to kill the opponent unit?

Yes — the +2 applies. Master Yi (Wuju Bladesman, Starter) is a passive "while" effect ("While a friendly unit defends alone, it gets +2 Might"), not a triggered ability, so it's active the entire combat with no window to dodge it. As long as the defender is the only friendly unit at that battlefield, its Might is always +2 — including when Ambessa's "When I attack, kill an enemy unit here with less Might than me" ability goes on the chain and when it resolves.

For the math: your Empowered Ambessa, Respected and Feared has Might 5, but while attacking she gets [Assault 2] (Rule 807), so she's at 7. The kill only happens if the defender's Might — including Yi's +2 — is less than 7. A 4-Might defender (6 with Yi) still dies; a 5-Might defender (7 with Yi) survives, where it would have died without the legend.

One caveat: if the opponent has any second unit at that battlefield (or you bring a second attacker), the defender is no longer "alone" and Yi's +2 stops applying immediately.