Pairing Routine, or if you're good enough at planning enemy movements ahead of time you can do threatening, with a level 10 Bard makes earning elemental challenges easier than ever! Zither has the fastest, most spamming ability (even more than an elementalist with 60% crit chance), and most damage versatility than any other class before it. Aside from spirit damage or poison damage, which you AWs (sob prior patch) or elementalist (cheaters) should already have done, you can get the easiest kills without even minor cool downs making bards hands down the best for these challenges, sorry people who earned it prior for Electric/Fire/Cold.
For those who haven't noticed it yet, you're welcome, for those who have already farmed it, I'm sorry, and for those wondering why I'm spewing the new common sense, well why not

.
If you're wanting this portraits, banners, and titles then become an offensive bard today! (
guide for offensive/support bard)