For the one in the Sunset Station Tunnel, go into Final Form and use Reflega above its head, and when you fall down after using it, just spam Reflega and it should be defeated easily. Make sure you have Reflega on your quickset menu.
For the one at the Old Mansion, just go into Wisdom Form and use Shot. Don't use it so much as to where it's being spammed, because if you go into the finisher, it'll take too long and you will probably wind up having more trouble than you should with this one. This one really isn't that hard if you keep on trying.
And for the one in The World That Never Was, go into Wisdom Form and use Shot. Equip Fenrir and unequip any Combo Plusses that you have, and equip any Combo Minuses you have. This will make it easier to get to the finisher, and you'll be able to attack faster. You have to take care of the Reaction Command well, or you won't get rid of this one easily. While pressing Triangle to do the Reaction Command, slide around using the analog stick at the same time, that will make it easier for the "Hide" Reaction Command in. Once behind the Mushroom, start shooting. Remember, you can only get in a max of 9 hits each time you go through the Reaction Command cycle.
That's how I beat those Mushrooms.