Get at least one life repelinser stored to use in the middle of the battle, three special moves stored (use one right after the other - just press R2 repeatedly, you don't want to stay close to Mariska for too long)the special moves will cut down her health to about half, and remember to roll out of the way, or dodge her at all times, keep her at a distance, and use the shotgun.

if you are using the shot gun stop using it and use your pistols, basicly you need to
watch her every moove and eventually you will catch on to her attacks, get as many
restoring health thingies as you can if you have never beet her before. never stop
shooting her no matter what. use your power up for your pistol and keep shooting her.
Trust me i know what im doing i've beet the game twice in a few hours.

kill the 2 girls surrounding her and then she transforms use the shotgun and tojo blades
jump over her when she dives