Farming MU Legend Zen Tips
The only way to gain zen is to spend less than you get. The easiest way is to make a rate based character, like AE, AGL, or ABK with low wings and weapons, or afk in a spot where you can kill the mobs before they even hit you. That way all your zen doesn't go into repairs, only into the bot. For example, on my agiBK I gain 10m per day in IK hotspot, but would lose zen in the DIK hotspot because they would hit me more often.
Since zen is based on exp and the bot priced on level, the fastest way I found is to make a DL, and lvl him to 200ish in a zen set. Then delete the character and repeat. I used this method when I used to play CC.
There isn't any legit way to get enough zen to buy tons of elemental runes overnight. Well, unless you at market selling your stuff for zen I guess. The way they used to buy MU Legend zen is to bug the web shop to get infinite goblin points, then they would buy the +13 wings in xshop and NPC them. I think they patched that though, otherwise no one would tell how it was done.