If you are a low level, you should hit dummy! If your level is 5-10, you should start killing cow or goblin. When your level is high enough, go and kill hill giant and hobgoblin.
You should train your lowest level
for ex. if your attack is 2 and your defence is 1, you should train your defence.
Hill giants will be a good idea.
I am lvl 68, hope I can help you!