I wouldn't really say broken; Fire-type and Flying-type attacks already have a high chance of KOing a Parasect in a single attack. You can't really do more damage than a straight out KO.I'd say that whether a dual-type attack is plausible depends on whether its constituent elements overlap. For example, if Heat Wave is Fire+Flying then what happens if you use it on a Parasect? Fire+Flying vs. Grass+Bug = 16x damage? That would be obviously broken.