So those obsessive Japanese have figured out how to force grotto bosses to give up their rare/common items.
WARNING
Only cheat if you like cheating or maybe just want to try it once. It's your own fault if you cheat and it sucks the fun out of the game for you.
WARNING
1: Deliberately get your butt kicked until you're low on HP (you must be down more than five complete heal spells).
2: Make your way right up to the grotto boss.
3: Quicksave.
4: Turn off your DS (this is important).
5: Go to the online "Hoimi Table" here
http://www20.atpages.jp/dq9tools/hoimi.cgi
6: Turn on your DS and load the quicksave.
7: Pick a character who knows the basic Heal spell, has some MP, and preferably has a low Magical Mending stat. Check what their Magical Mending stat is exactly by looking at their attributes and write that number in the first box of the page.
8: After entering your stat, click the button beside the first box, and the lower second box will tell you about how many HP that person's spell is expected to heal.
9: Find someone who is hurt a fair bit more than that amount, make a note of their HP (on paper or whatever), heal them once, and make another note of their new HP. Subtract one number from the other to see exactly how many HP they were healed.
10: Write the healing amount of your first heal spell in the box numbered 1 (the third box).
11: Repeat the healing and note-taking four more times (don't max out a character's HP, because you need to know exactly how many HP were healed). Write in the HP amounts in the boxes numbered 2-5. Don't bother healing any more or using boxes 6-10.
12: Click on the button at the bottom of the page. The calculator will read the "randomness" of your heal spell amounts to predict the variables in your game.
13: You should be taken to a page with links that say Javascript or CGI. If there's more than one link, you may have to heal a sixth or seventh time and enter the results to narrow the search. Click on one (I suggest opening it in a new tab in your browser). It should show you a chart.
14: If you used the heal spell five times, you should be on space number five (there isn't any obvious indicator for this). That means you're in space number five of the zero column. In order to get a rare drop, you must fight the grotto boss while you are in the space directly preceding a red square (blue is a common drop). Every time you heal (or even attempt to heal) with any character, you advance one square. So, manually count off how many squares you would need to advance, heal the appropriate number of times to get to that square, and then engage the grotto boss.
15: You can use heal or any other spell in the fight, but DO NOT use a Coup de Grace, should one become available.
16: Win the fight. Be rewarded with the rare drop if you did it right.
17: This apparently also works on rare alchemy success too, but I haven't tried it.
Edit: Youtube is also helpful.
http://www.youtube.com/watch?v=NiDAwt7n3eA