What to do if another user, or users, disagreed with me and serially downvoted bunch of my posts without reading them (all votes were cast within minutes)?
Based on the meta.SE question “What is serial voting and how does it affect me?” and the very thorough accepted answer:
Do nothing. Automatic script checks for and detects unnatural voting patterns, and reverses serial up or downvotes after 36 hours or so (it runs every day at 03:00 UTC).
If after 36 hours situation yet to be rectified, flag one of the posts for moderator attention, and explain the situation.
My advise will be - do nothing. Stay calm and keep on moving. Imagine that you have, say, 200 answers (which is quite a lot), everyone of which had been up voted at least once. So, you have 2000 points - I assume, your concerns are about reputations, since otherwise there is not point in worrying at all.
Imagine now that someone is in bad mood and down voted you serially for some reasons. Then, actually, all damage such person can produce is about 400 points. Even if there was not a system for detecting serial down votes (and there is a system, as you've already mentioned and it works pretty well), even in that case it is not a big deal.
I believe that concentrating in questions and answers in ant case will give one incomparably more profit than trying to seek revenge. After all, remember that one up vote can be annihilated only by five down-votes.