Try going to the Black Omen.
There are three time periods where you can go to there. 1,000 A.D., 600 A.D., and 12,000 B.C. It's important to start with 1000 A.D. and make your way back, because if you beat the Black Omen in 12,000 B.C., it won't be there in 600 A.D. and 1,000 A.D. since you already beaten it (you've altered the future by changing the past!).
Once you've beaten all three Black Omens, you can stay in the last time period (12,000 BC) and fight random enemies inside -- making use of the first save point there. Just can move back and forth, save your game, and then go back for more.
As a side note, your inactive party members will only gain 3/4 EXP, but if they were to gain more than one level as the result of the EXP gained, they will only gain one level (no more EXP will be carried over).