You can combine two seals at once to make rare seals like Entity, Tempest, Tundra, Etc. To do it you must take a Coin and (EX: Grulla) and engrave the Earth seal and Mistral Seal and it will have the Same effect as if you used an Entity or you could use the Wind Seal and the Land Seal insted. You can remake any rare seal by combining a coin with the first seal(Earth,Wind,Fire,Etc.) of one of the Triangles and the Potency seals(Inferno,Mistral,Land,Etc.) of the Other point of the Triangle. the Triangles work like This:
Fire Earth and Wind by using one of there coins plus a L1 Seal of another side and a L6 seal of the last side.
The other triangle is: Water Force and Lightning.
Just go one of the Towers. Equip with as many Cure, Cure Re, etc. as you can. Kill all the bad guys except one. Now, when you hit the last bad guy, when the power gauge comes up, press the button straight away so you don't damage him too much. For each hit, you get a few EXP and DEV point. Heal the bad guy if he is about to die. Keep doing this and you will level up in no time. Since all you need to level up is 100 points, unlike games like Final Fantasy where it gets progressively harder to level up the higher you go. I managed to get to level 50 even before I got to the tower of water.