First, get at least two statisticians and put them in the Innocent farm. Once they start making more, you can take them out, put them on an item, and with someone in the hunter squad, go through it and clear them out (it's best to put them on a low level item, like a thimble, so that you won't have to deal with the other enemies or risk them taking the statistician out). You can also just max out the Farm so that the offspring are already subdued and basically just let them brew and multiply. Once you get enough to combine to make a level 900 statistician, you're done. Put it on any piece of equipment you want (emblems are great for stuff like this), and you're good to go.
Second, make a Sage. You'll want one for the post game.
Go through MT1 on 1 star until you're well past level 300. Make sure you have the experience maxed out as well, forgo money, weapon experience, and skill experience for now, there's a money map that'll handle weapon/skill experience and money is a non issue with Carnage Dimension.
Once you can beat MT5, use that to level up until you're about 2000~4000, depending on how many extra maps you've cleared, and then see how you fare against the Carnage Dimension map. To unlock it, you'll need to complete four or five quests from the quest shop which include beating an Item God 2 among other things. Once you've accessed the whole Carnage Dimension (which is an alternative to the regular stages with enemies that are about 100-1000 times more powerful), try the earlier stages to get some Carnage weapons, which are vastly stronger than what you'll ever have access to otherwise (a Carnage r1 weapon is far stronger than a regular r40 weapon). Once you've been outfitted with Carnage weapons, try your hand at the Carnage versions of the Martial Training stages and see if you can at least beat MT1. MT5 will take a bit more time and regular some playing around with, though the easiest strategy for it is to get some Carnage shoes that grant +3 MOV and equip them to a Sage, move her to the top of the tower, and then just pelt the foes with Tera Ice.
The 100 Asagi stage is also good for grinding, and is a bit more manageable than MT5 in carnage dimension. See how far you can get with the sage and do some compare and contrast with the Asagi's and your Sage. If you need to, power up Land Decimator. If you use the Money Map by RozPower, you'll max out your weapon/armor and your skill experience in quick time (as long as you have them set to give you experience so tone the mana, money, and exp down to 0 for the time being). Using the Tri-colored mochi will help recover SP, as well as any curries you have. Once you've maxed out the skills' levels, start powering them up a bit via the skill shop so that they're more potent. For the Sage, you'll mainly want to use Tera Ice and Land Decimator as the first set of enemies on CD MT5 are a pain without Ice and the Asagi stage is a gruel fest without LD.