I would say it is a medium difficulty, but you can adjust that in the options menu. You can even use a cheat by pressing F1 key. That will kill the enemies instantly in case you want to skip a specific battle.
Viewing post in Streets of Ryona ( hentai streets of rage version) comments
Oh I see, well in that case you need to learn some combos with the specific character you pick, like "v + >+punch", or do that twice quickly for a special move. That will help you survive the waves. It's not a easy game indeed, the difficulty was adjusted for a intermediate to veteran player, but it's not hard at all.