Tips on how to get your Mercs *almost* all to level 9: Don't waste money upgrading Mercs you will change later. Don't keep buying Mercs, just what you need. Don't keep rerolling the selection, maybe just a few rolls to find what you want. Use up all your 3 lives, each loss gives money. I got 4 of my mercs up to level 9 and one at level 8. Enjoy. :D Oh, and, vote up if you THINK this is helpful. :D
Rogue level 3+Boots of Recovery+Belt of Recovery+Cloak of Town Portal=Awesomeness.
Mine was able to survive from floor 1-7 since I only have 7 levels atm. It leveled from 3-6. XD