If the damage done to you by the monsters in an area is small (for example, 1), hold Jand allow the game to idle to gain levels. You can also do this to grow levels for your armor and Zanpakutou. To reach level 90, if you have good defense and the damage done to you by those enemies next to the town (first street), you can use that strategy. If not yet, you fight the Bleaches using the previously described strategy above to gain levels for your Zanpakutou and armor. After that, check if you are now strong enough to leave your game unattended for the easy experience.
During the mission to go to the real world and kill Mechen, you can fight Urahara (level 90). Defeat him to get a Strength Ring. Sell it to get Nan G to purchase anything desired and never run out of money.
Get the Nan G with the "Easy money" trick, then purchase 99 Steaks and 99 Blue Bottles (Large). Use them in Boss fights or when facing very strong enemies. You will almost never die, unless the damage is larger than your HP. After taking plenty of damage, use the Steaks to heal yourself and Blue Bottles (Large) to refill MP.