What merc to use with a javazon?
Your average java is better of alone, most of them stay in the cow lvl 99% of the time and the key to killing fast there is herding the cows. A merc will break up the herd so they will help the cows and not you.
A might merc, act2 nm offensive, is a good bet if you have a bow on switch. They can also deal with Immune to lightning, but they tend to need lots of help to stay alive.
For a pure Javazon made to play in any area of the game I would stick to a holy freze merc, act2 nm deffensive, They have decent damage with a elite spear/polearm, hard to kill because they freze all the badguys and gives you more time to kill monsters. However they do make it very hard to herd monsters into large groups.
Barbs as mentioned in a post above are nice tanks.
Act 1 rouge merc can also deal a fair bit of damage with a +3 skill lightninghose or a good bow. However I feel they are a bit fragile as a merc for a Javazon.
Also you should remember that your valkyrie and decoy skills can be used for many of the same purposes as other chrachters use mercs for.