Keep your cristals until you have 750, then buy a premium pack. It's much more profitable than buying scrolls one by one.
Your first goal must be to reach the fairmon region by following the story. By then, you will have received a water magic knight, which will be very usefull to you. So keep her and up her as much as you can.
Some 2/3 stars monsters are better than 4 or even 5 stars : water howl (Lulu), one of the best heals you can get for free. You can get rid of the fairy you get at first. Fire harpu, wind warbear, fire inugami (you get him in the fairmon region I told you about), water garuda, wind pixie, wind griffon are monsters you'll get pretty fast, keep them and raise them to 6 stars, they are all worth it.
Your second should be to fuse Veromos the dark Ifrit. You'll find
here the monsters you'll need for the fusion, most of them can be found in secret dungeons. This website contains a LOT of valuable information.
Do the daily quest every day if you can, your own level will grow faster. Some content, including what the store offers you, beeing level related, it's pretty important. One of these quests requires you to fight friends, it's badly translated. It means you have to use 3 friend monsters a day. These quest will require you to go to Cairos dungeon, do it as soon as you can.
And if you need more precise advice, I can have a look at your monsters and help you.