My suggestion is to add a quest (It should be available to take once per week).
This quest can be taken by any Honorable warrior (11 H.R. or higher Procyon/Capella) from the Port Lux officer to prove that he is a Hero and that his nation is better.
The quest name should be something like "Path of a Hero".
There is no lvl restriction for this quest.
Quest is same for any lvl, nation, H.R., etc.
Quest is only Soloable.
There should be statistic on the site main page that show who is the best (and also it would be good to add same for NationWin at once), it should look like this:

>>From the left there is nation war bonus, from right hero bonus.
>>Red colored name of server show you that Procyon have bonus, green that Caps.
>>White numbers show how many weeks in row Winning nation has a bonus (same for Hero)
>>Yellow numbers show how many weeks in row same Hero is the best one and give his nation a bonus (so if Hero change his nation and win bonus for other nation his counter doesn't stop [I made such thing for the Mercury on picture, so that everyone of you could understand])
Yes, there should be a bonus for nation which has the best Hero, this bonus will give +10%drop rate +15%alz drop amount +50%2 slot item drop.
Also it will be good if The Best Hero (the one who got bonus in a row longer than other) will be recorded somewhere on site, so if even he will not become best one some week his name will still be known as the best of the best 
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Now lets closer look to the Quest itself:
So you get the quest from Port Lux officer Morrison, you can go in that Arena whenever you want, but you must finish it till the end of the week (I mean the same end time like in TG 150-170 when the bonus is given), if you will not complete it to that time it will reset and you will need to take it once again, if you will be in Arena in the time of reset you will be automatically thrown from the Arena and the score you taken will not be considered the quest will reset.
Now look closer to map:

You will spawn on the red dot in the center of the Arena.
You have 1 min to prepare (it will show timer same like on TG from 1:00 till 0:00, after that time it will disappear).
You are not able to move on black and purple territory (Black territory must be something like Abyss in nowhere).
You are able to move on Yellow (some king of sand or whatever, it's up to developers) and on blue square).
From the purple territory from 4 ways will spawn mobs - 40 mobs of same type from 4 ways (in tottal 160 mobs).
They will spawn one after another each 2 seconds and will walk right to the blue square.
The blue square is the thing you should protect, it has 10 lives, you must stay right on it (it should have the length from it center to each side at least same as the longest range of mobs that will spawn there, so that it will be fair to malee classes).
If you move from it to the yellow territory (so you're not on the blue square) every mobs that will step on the square will disappear and will -1 from your life counter.
For every killed mob your mob kill counter add +1 (in the end this will be the score which is counted to compare other ppl)
The mob spawn starts from nipperlug and goes until Leth Tyrant (or any other, I mean the last 160 mobs should be the strongest in game, so that it will be impossible to kill them, and as I haven't been in every dung I can't say which one is the strongest but I think it's Tyrant).
Mobs drop the same drop as the real one on maps and dungeons (as for dung bosses they shouldn't drop chests cose it will be too imba, give them drops compared to bosses in maps and strong mobs).
Also as you can see there are 4 green dots => those are NPC.
To 1 NPC you can sell something or buy HP/MP pots (same as in normal maps).
2 NPC is WH (Agent of Port Service).
3 NPC sells you resurrection stone (1kk each), this stone resurrect you right after you die in blue square after 3 seconds with half hp/mp and without buffs (you are not able to use Odd's in this Arena). You can buy up to 5 resurrections, but you can carry only 1 at time, after you resurrect you can buy another one.
4 NPC clear for free (after you press "Clear Arena" dialog button) all Arena from everything that lie on it (include alz and non drop items - so everything). Also you can press dialog button "Start immediately" to start immediately next round without waiting time
After each 160 mobs round you have 30 sec to sell to NPC or to store to NPC items that you have dropped in the round, or to buy pots...
You fight untill you can (cose as I said, you can't kill 160 Leth Tyrant solo xD at least now).
After your life counter is set to 0, statistic of arena show you how much time you have spend, rounds you've cleared, mobs you've killed.
The amount of mobs killed by you (disappeared mobs that -1 your lifes are not counted) is counted as your score for the best hero of your Server.
If you become the best hero in your server your name will be on the front page of Cabal Online, you'll have bonus as every other person of your nation, also you recieve the title "Hero" which should give same stats as title of Bringer[HP UP +500, Resist Critical Rate UP +20%, Resist Critical Damage UP +60%, Sword Skill AMP UP +10%, Magic Skill AMP UP +10%, Critical Rate UP +5%], I think it will be fair.
Hope you like it, cose I can't weight Arena like that to be added in Cabal, offc I know that I will not be the Hero, but maybe sometime I will))
And it will also bring a lot of fun, alot of EXP(therefore lvls), a lot of new items and alz in-game and bla bla bla 
Feel free to ask or suggest something.
FAQ
Code:
Q1:if any hackers enter this, economy will be ruined by them
A1:Have you read all of the text???
First of all, you need 11 H.R. to enter.
Second is that it is solo, and the score is counted, if anyone will finish this Arena he is 100% haxor (got a ban).
Third, it is only able 1/week.
Fourth, mobs drop the same as simple map mobs.
There is no point to make bot/hax for that Arena.
It's way better to make bot/hax for solo FT2 => way way bigger profit.
Or make 24/7 spam PF.
First read, think, then post...
Q2:Is there a short version?
A2:Hmm, if you mean a short version of a "wall of text" than no, cose I made it as shortest as I could )) Every line give some information...you can read without "edit"line (last one) and paragraph above it ;)
Q3:Nice idea, but it would definitly need some changes regarding the different classes. Since theres a litle diference between FA and WA getting hit from 4 sides at same time.
A3:Well, since:

Originally Posted by
KAMIKADzE
The blue square is the thing you should protect, it has 10 lives, you must stay right on it (it should have the length from it center to each side at least same as the longest range of mobs that will spawn there, so that it will be fair to malee classes).
You can slide with your WI/FA through the whole perimeter as much as you want while shooting mobs/RB from the range, but malee class need to hit 'em in close combat...so it how to look for which classes it is worser ;)
Q4:But the reward is definitely to be changed, or we will have even more nationhoppers :/
A4:Still a bit of drop rate (+10%drop rate +15%alz drop amount +50%2 slot item drop) is not teh awesome, the main reason of nationhoppers is not a 10% drop+exp+axp+petexp but the port to berdy and dungs, so the bonus that didn't give warps almost will not have any affect on nation changers.