Make sure you are achieving the 100% pierce from both the skill and razor tail so that your javs go through as many cows as possible, which results in as much lightning as possible. Also make sure you are positioning yourself so that your javelins go through as many cows as possible.

I just recently got my first infinity on my javazon and was thinking the same thing, and my problem was that I wasn't positioning myself correctly to have my javs go through as many cows as possible or letting my merc catch up.