You can earn Nexon Cash by anchievements.
Here's the list of anchievements:
Normal Achievements
1) Finished the Apple Training Camp (250NX)
2) Finished BossQuest for the first time (17500NX)
3) Killed Anego for the first time (3500NX)
4) Reached level 70 (5000NX)
5) Reached level 120 (7500 NX)
6) Killed their first boss (1000NX)
7) Equipped their first Dragon Item (3000NX)
Reached mesocap (2,147,483,647 mesos) (10000NX)
9) Reached 50 Fame (2000NX)
10) Killed their first Papulatus (2500NX)
11) Saw a GM (500NX)
12) Successfully scrolled an item (1000NX)
13) Earned a Zakum Helmet (2500NX)
14) Said "cc plz" (100NX)
15) Flew to Victoria Island by Shanks (Southperry) (500NX)
16) Killed Zakum (5000NX)
17) Completed a trade (250NX)
18) Killed a Snail (100NX)
19) Killed a Pianus (2500NX)
20) Hit more than 10,000 damage to one monster (3000NX)
21) Hit 99,999 damage to one monster (6000NX)
22) Reached level 200 (35000NX)
23) Won the FoJ (Field of Judgement) (3500NX)
24) Created a Guild (2000NX)
25) Completed the Guild Quest (3000NX)
26) Killed Horntail (30000NX)
27) Complete Kerning City PQ for the first time (3000NX)
28) Complete Ludibrium PQ for the first time (3000NX)
PQ (repeatable) achievements
29) Complete Kerning City PQ under 5 minutes (400NX)
30) Complete Ludibrium PQ under 10 minutes (400NX)
31) Complete Ludibrium PQ Stage 8 under 90 seconds (250NX)
32) Complete Kerning City PQ Stage 4 under 1 minute 15 seconds (travelling time) (250NX)
33) Complete Kerning City PQ Stage 5 under 1 minute (300NX)
34) Complete Ludibrium PQ Stage 9 under 90secs (300NX)
35) Complete Ludibrium PQ (clear stage 9) (100NX)
36) Complete Kerning City PQ (clear stage 5) (100NX)