I’m sure there are a thousand different ways to gain levels quickly, but I’m going to tell you the easiest ones that I’ve found.
The first one may be a little difficult for some players. You need to have someone who is quite a few levels ahead of you. That’s mainly the trick, tag along with them and gain all the experience you can. The cool thing about Phantasy Star Online is that all you need to do to gain the experience of killing an enemy is hit them once. So if your able, stand back and put a few bullets in the enemies while your higher level friend takes them all down. You’ll be high leveled in no time. Forces can also use magics on the enemy such as Zalure (which does no damage to enemies), some of which will allow other members of your party to also gain the experience from the enemy.
The other easy way is to take a previous mag with you. A high level mag from an old character can make you nearly twice as good as you would normally be. Mags actually give quite an increase in statistics so don’t be afraid to trade your Mag off to a lower level person and then get it back afterwards. It really helps.
Then you may be thinking, so what? Am I screwed after I have to give the mag back? No. The whole time you’re using the higher level Mag you should be leveling up the one your already given, so when you have to give it back you’ll have a somewhat moderate replacement ready to go.