The Plaguazon

The Plaguazon is a very viable build in both PvP and PvM. I personally have seen the success of this build in both PvP scenarios and soloing the game in PvM, even hardcore. There are three keys to the viability of this character--increasing your damage, choosing a method to whittle away your opponents' health while they are poisoned, and finding a means to delay your own death while your poison works its magic.

The first key aspect of a Plaguazon is increasing your damage, as it is often the case with many characters. To do so is fairly easy--increase your Plague Javelin and Poison Javelin damage. Any items with + to skills and +% poison damage will help. Keep in mind that when using Plague or Poison Javelin, the javelin never actually hits your opponent--it is the skill that does the damage. With this in mind, things such as poison damage charms, %ED, or anything that would increase the damage of a "normal attack" do not apply.

Secondly, you must choose a method of doing damage to your opponent while they are poisoned. For this, many choose the lightning javelin attacks--I personally would suggest Lightning Fury as it is ranged and this is extremely beneficial, but Charged Strike is also viable. The reason behind choosing a lightning javelin attack is simple--you are already trying to maximize your poison javelin damage via items that offer + to skills; these items, whether they are "+X to all skills" or "+X to Javelin/Spear Skills", will boost any javelin skills that you choose. Any other method, such as using a bow on switch or using skills such as Jab would require you to boost another aspect of your character, such as raw, physical damage. With Lightning Fury or Charged Strike, you will also need to worry about IAS; this is very beneficial to your character, as being able to shoot off a few Lightning Fury while running circles around your poisoned opponents is a general and simple strategy one should utilize.

Lastly, you must find a means to, well, not die. Since poison has a duration (Plague Javelin has a shorter duration than Poison Javelin; the latter does more damage overall though), you must keep yourself alive until the poison eventually kills your opponents. There are several ways of doing so--you can choose to tank your opponents, which will require boosting your block to 75% and utilizing Dodge/Avoid/Evade. This is the most character and item-intensive option, but the one that is simply the most beneficial in PvP; in PvM, you could choose an alternative. Manually evading your opponent is also an option; use a Valkyrie to distract enemy monsters, a very cheap and easy alternative. Faster run/walk can also be utilized to great extents; simply running circles around your opponents is another easy alternative.

Of course, the most effective strategy and way of increasing the effectiveness of a Plaguazon is combining multiple strategies. In PvP, Plaguazons require high poison damage, fast attack speed, fast run/walk, 75% block, and high Dodge/Avoid/Evade; this way, the character can tank the opponents it is possible to tank, run circles around the opponents that are more difficult to tank, and all while being extremely effective with both Plague and Poison Javelin and Lightning Fury.

So, a summary:

Skills:
20 Plague Javelin
20 Poison Javelin
1+ Valkyrie
1+ Dodge
1+ Avoid
1+ Evade
20 Lightning Fury or Charged Strike
XX Lightning Fury/Charged Strike synergies

Items:
+ to skills
% Increased attack speed
75% block (optional for PvM; 50% block is also viable)
% Faster run/walk (optional)
Max resists (a given)
% Damage reduction (optional for PvM)

Strategies: (requirements)
-Tank your opponents (Dodge/Avoid/Evade and high block)
-Run circles around your opponents (Fast run/walk)
-Distract opponents (Valkyrie)