Some tips:
Join a Clan (lvl4+): This will help you so much. AFK farm mobs that drop Proof of Bloods, which you donate to the Clan and get Coins. Use the Coins to buy XP (15 Coins for @2%, 70 Coins for @33%). A high enough Clan can also use a Fireplace in the Clan Hall which will give a huge amount of XP in about a 30min period, the more people around, the more XP you get per tick.
Pay attention to the Public channel for people using Boss / Rares summoning crystals. These instances last for 30min, with a Boss that will always drop its Core, after the boss is dead, there will be 15 waves of adds, these have a high chance at dropping the same Cores. Focus on Bosses / Rares which add to your P. Atk / M. Atk or Crit (check in the Codex). Easy way of getting Cores (much much more rare to just farm normal mobs).
Gear: Save any A Grade gear you get and lvl it up, even if you're not using it (but only stuff you can use). When you can, Combine two A's into S, by doing this you have a chance at getting a "rare" version of that gear. Do the same for S Grade gears; but only Upgrade whatever R Grade gear you have. This is what you'll be doing at high levels.