My tip would be to grind it out a bit. Maybe your level is too low to fight Rusalka or DeRosa. I suggest you take them on around levels 30~35 for an intermediate level of fight. A trick on fighting Rusalka would be to use Lightning/Aquatic Slayer/Area of Effect attacks. Remember to be wary of its charm and burst damages. Also default when Rusalka hides to avoid team wipes.