General

Use the mesos you have to fund the following link mules up to level 120.

Kanna - 10% dmg
Demon Avenger - 10% dmg
Demon Slayer - 15% boss dmg
Xenon - 10% all stats
Luminous - 15% pdr
Phantom - 15% crit chance

Then focus on the char cards. Cards like Kanna and Demon Avenger give 3% boss dmg just for using them. Combine as many of the lvl 100+ char cards as you can into sets...having a set of 3 where each char is over lvl 100 will get you like 5atk and 3% boss dmg for each set. If you have multiple pirates it's good to organize them into their own set which will net you 6% pdr.

Not much else you can do with 300 mil...thats basically nothing right about now.

Get involved in Professions. I highly suggest getting the Alchemy Profession. Work carefully to level this up past level 10, then to Master and finally Meister Level. At this point, go to random maps that you know to be very active with people training, even maps frequented by botters. Look around and you'll find mysterious herbs and ore veins. Mine or collect these...by doing this often when you have free time you'll acquire many of the items needed for Alchemy for free. When you need specific Herbs in bulk and feel like farming them, the best option is to go to Evo and attach the cores specific to what you're looking for. Buy a few Lucky Winter Drop coupons. Also, if you've been doing MPE fairly regularly to farm green and red mpe potions, you'll find eventually that you'll get a Greed pendant as a drop. This will give you a significant boost to your drop ratio as well. Combining these together will acquire for you a significant number of herbs.

Some recipes that sell well: Make Advanced Hero Potion(10% atk boost) and Advanced Boss Rush Potions(20% boss dmg). These sell fairly well over time and for a very high price. Furthermore, if you boss often, particularly Zakum, you'll likely occasionally acquire some EXP Potion Recipes. Once you've acquired a ton of the herbs necessary(Juniper Berry Seeds) make these en masse. Remember that once you click the recipe and learn it...it will disappear in 24 hours. Now start mass making these potions as much as you can till it expires. These sell insanely well in the FM.
You can also make 12 slot bags as a Meister level alchemist...from my experience Mineral Bags sell for far more than herb and much faster as well. Do this often and consistently, maintaining yourself at Meister level at all times...and you'll slowly but surely build up a great amount of savings eventually.

Goodluck