In order to be considered a 'Master Romancer' for a species you'll need to have 7 of that particular species in your garden.
An easy way to do this achievement would be to become master romancer for the White Flutterscotch, and then become Master Romancer for the variant flutterscotch's.
Varients of the White Flutterscotch are actually considered different species, so you can get this achievement using only variant flutterscotch's.
The only things you'll need for the Flutterscotch to mate is for them to eat 1 piece of fruit, and have a house, so it would be alot easier to get them to mate than other species.
Each variant of a White Flutterscotch counts as a new species, so you'll need to make a total of 35 White Flutterscotch's (doesn't have to be all at once) and then make 5 sets of 7 variants to get this achievement.
Or you could even make 63 flutterscotches and make all 9 sets of 7 (also does not have to be at once) to get yourself closer to the 'Master Romancer' achievement. Then you could sell all of them to make yourself some money, or if you decided to buy the flutterscotch's you could get alot of your money back.
I would suggest doing this one variant at a time. Make 9 white flutterscotch's, and then buy 7 of whichever item you need for the variant and change 7, then make another 7 and repeat until you've done all the variants.
Example: Make 9 White Flutterscotch's, buy 7 poppies, feed 7 of the white flutterscotch's poppies. This will give you 7 Red Flutterscotch's and the 'Master Romancer' Award for that species, then make 7 more White Flutterscoth's, buy 7 watercress, and feed 7 of them the watercress, this will give you 7 Green Flutterscotch's, and the 'Master Romancer' Award for Green Flutterscotch's. Now repeat the process for the rest of the variants.
Red Flutterscotch - Have a White Flutterscotch eat a poppy.
Green Flutterscotch - Have a White Flutterscotch eat a watercress.
Blue Flutterscotch - Have a White Flutterscotch eat a bluebell.
Pink Flutterscotch - Have a White Flutterscotch eat a water lily.
Black Flutterscotch - Have a White Flutterscotch eat a tulip.
Brown Flutterscotch - Have a White Flutterscotch eat a bullrush.
Yellow Flutterscotch - Have a White Flutterscotch eat a buttercup.
Orange Flutterscotch - Have a White Flutterscotch eat a Bird of Paradise.
Purple Flutterscotch - Have a White Flutterscotch eat a thistle.
I take no credit for this variant list, it came from a very helpful Viva Pinata site.http://pinataisland.info/viva/Main_Page
Hope this helps any of you. I know this works as I had Master Romancer for White Flutterscotch, and when I became Master Romancer for Orange Fluterscotch I got the achievement.
There are 9 different variants of the Flutterscotch so you can also become Master Romancer for 9 species this way, that only leaves 11 species to get the 'Master Romancer' achievement.
EDIT: Just realized I hadn't counted the white flutterscotch itself, so using this method you'll actually get 10 species as master romancer.