Snake has bloodthirst or bloodliust, one that gives it more attacks if it kills one, so do NOT use clones/summons, apart from that there is no trick to it, just need like 20k power or so.

I passed it at just under 19k with lightning, Cow Zabuza/Kurenai/Kimimaro, saw video on youtube, only took about 4-5 tries.

It's not a bug Kimis block is not 100%, especially if enemy has much higher stats. This will be very common in ninja exams, get used to them going through your Hinata/Kimi lol.