An easy way to defeat just about any boss without getting scratched is easy. All you have to do is pump up your earth spirit to at least level 36 so you have the magic barrier. Then other destructive spells if you like. Apply the magic barrier every other turn. If you do this nothing can hurt you, not even Mammon. Then simply attack with anything you have the turn after you put the barrier on. Another tip don't save at the floating monastery. If you do you are stuck in that infernal place forever.
There is a great way to increase your HP, MP, Defense and Agility all at the same time. First you need to go to Cull Pass. Make your way through the level until you get to the last semi-large room. (There should be 2 exits, one that is blocked off early in the game and one that leads to Normoon). The only creatures you should encounter are Blood Jell's. When you fight them be sure to do it close range (with club), (this will boost your HP percent), stand still when they cast homing arrow, the ones that hit you will boost your defense and the ones that miss you will boost your agility. After each battle cast heal on yourself, (this will boost your MP). Sometimes this method is effective and sometimes it isn't. So keep at it and you will defiantly boost your stats quickly.
When you defeat Mammon at the end of the game, you end up at Epona's house and cannot do anything after you exit, no fighting, traveling, etc. So I suggest not beating the blue hairball so you can enjoy kicking all the other enemies and raising your stats to the max (elements, health, magic, defense, agility). Show the 300+ HP enemies who's boss without using Magic Barrier or Healing (until after they explode). Then come back when you've had enough and smash him to the dirt.
When you get an element press all four buttons at once, this takes a lot of practice but it should give you an element in each category.
In the Desert, there is a treasure vault. Once in the barren part of the desert, where just enemies attack you, go South, then East. Check your map frequently. Once you're in the middle of the desert (check your map) you will see a giant black diamond bobbing up and down in the sky. Move towards it and you will see a pyramid. Go to the top and there will be a warp block. Stand on it and you will be transported to the top of another pyramid. A castle will be in front of you. Go inside of it. Follow the trails inside and you will come to a room with a whole bunch of treasure chests in a circle and two spirits. This will boost your inventory way high.