The difficulty to get a reaper depends on the map you're playing - oh and your difficulty makes a lot of difference - it's balanced for wild lvl 120-150. Island is the best for Extinction Core, but Dead Island is smal, what means it's sometimes a little tricky to deal with zombdodos and other Creatures there. I think, gigas, lightning dodorexys and others like that are to weak to have a real chance - sure, it's possible, but you must accept the higher risk that something goes wrong and you die. Killing Zombdodos? Bring baby food, blood and fortitude vials, A-Wyvern (Ice is best), A-Rex, A-Spino... better taming a boss first can help you a lot. At the end - find a reaper queen and clear the area arround her by running and biting, be sure not to bring a light pet on your shoulder, so the queen gets less damage than the craetures arround her and you should have no problems with not killing her while clearing. Lure her to the smal piece of land on dead island, where no creatures are spawning - you know - the smal place seperated from the main island by water (not the huge one with the rock in the middle). Now you can try to get an embryo. A shield, detox and bold vials will help.