To easily get an assassin or grandmaster ranking in mission 9 is to bring along a chameleon spell,if you don't have one go to level select and choose Rin and get Assassin ranking in level 3-1 or 3-2.

After killing the three enemies grapple up to the right watchtower and double jump to the cliff which is east to the health potion.

Now go a bit front and bring your map,now use your chameleon spell and go to the end of the level and Voila! There's your fire spell.

If you had two chameleon spells after you use one of them it will become one but after you assassin the level you will get another chameleon spell so you will get a profit because the chameleon spell will remain two but you will get a fire spell.

Level nine boss strategy:
Double jump to any of the watch tower and keep using a crouch attack when they try to get up.