Chapter 4 of Separate Ways has the objective being to "Stop Leon's Assassination".
Krauser and Ada meet at the island's radio tower. Afterwards, Ada gets a call from Wesker informing her that he's dispatched Krauser to kill Leon. Ada reminds herself that she doesn't always play by Wesker's rules.
Finding herself in the shipping lane from the waste disposal center, Ada goes on the offensive, through hordes of soldiers, even going so far as to sink the warship that Saddler has somehow recommissioned, all by herself (she even used a pair of Gatling cannons to destroy the warship's onboard artillery).
Arriving at just the right time of Leon & Krauser's knife battle, Ada shoots Krauser's knife out of his hand as he is about to stab Leon by the heart, and he is forced to flee.