So, the first achievements are pretty self explanatory, play a few games, explode a few clothes, and so on.
For the miniboss achievments you simply have to defeat the miniboss in the achievments name, each level should have a set miniboss, some of which appear multiple times, these are easy.
The achievements such as "Find the Clitorous" and the bike/item/ability unocks are all found just playing through the game, nothing special is required for any of those.
Getting 5 stars in one stage is another achievement, which should already have been completed as it is very easy.
Exploring all of hentalia means filling out the map, AKA finding all the girls, easy.
One requires you to enter gallery mode, which can be found on the main menu.
F. THE PLOT requires you to skip a cutscene in the game which appears after you finish a level, I suggest doing this on a level replay (Which you will be doing). (Press R or click the "skip" button in the bottom left on a cutscene)
SURPRISE! requires you to find a random encounter, which is really just clicking on the goblins which appear on the map in varous locations which is once again, quite easy. (Also gives you levels)
For all of the girl unlock achievements, that too requires you to just play normally through the game...you know all of these achievements are ♥♥♥♥♥♥♥ easy, I'll just skip to the noteworthy ones.

100%ing a girl means you must take off all of their clothes on a level.

For the ONE HAND achievements you are required to play through all of the girls again, but this time with ONE HAND MODE, which will be prompted when you select the level again.
For ONE HAND MODE I suggest using whichever bike you want, but I suggest using the SPEED GLITCH special, as it can really speed things up.
To get the achievement you must only complete the boss and do not need to get a 100% with the clothes.

That was really ♥♥♥♥♥♥♥ easy, just like all of the achievments in their games.
All of them are quite self explanatory, but hey, a guide is a guide.