For Lv1-50:
Stick with the 3 maps(Bloody Ice, Dessert Scream, and Green Despair)but i prefer to go to dessert scream.....than the other 2...

For Lv50-65 (50-55 and 55-65):
For Lv50-55 you can go get cabal alz some quests and if not kill some Stone Gargoyles....
For Lv55-65 you can go to the Golems or Dark Bladers... and never forget to get some quest..especially the STORY QUESTS....

For

For Lv.65-75....
You can go kill enemies or if you're rich to buy some many many cards...buy some MAP PARTS....and go there.....

For lv.75-80
You can go kill enemies also and if you have the money you can go and guy some Ruina Station cards...and go there....and i recommend you to have Party members....if you go there....

For Lv80-100(80-85, 85-90 and 90-100)
For Lv80-85, you can go and kill the weakest of UG's mobs....the LIHORN ZOMBIES.
For Lv85-90, you can go and kill the ZOMBIE WARRIORS, SLATERS, KNIGHTS AND OTHER ZOMBIES.....HAHAH LOL...
For Lv90-100, you can go and kill some DEATH SOLDIERS....

But OVERALL TIP FOR TACTICS: Taking Quest is the best way to level up easily.

Well that's all....Hope this will help you....