Euthanasia by hammer blow is a difficult thing to do. If you are training to be a carer there will be times when you will come up with animals that need this sort of attention. Run over by car is one particular injury that quite often needs to be treated like this, basically any type of gut wound that makes the gut exposed and opened will not survive.
Its a hard task but you can either end its suffering, or let it suffer. As a carer you should at least approach this subject, and if you don't have the stomach for it find someone in the organization who can be called on for this treatment.