Preds usually are pretty good, unblood or no. and to see the queen... without the helmet, he sees in the vision he was born with, and seeing as how Xenos are favorites for prey, the clan probably taught Scar how to see aliens in their actual vision. He's an Unblood, he's not perfected yet, and against a queen so soon, on his own, he may have really needed Lex's help, seeing how he's an Unblood, but the Queen is a very high class Xeno, being around for so long and haveing such a hive... Powerful queen, vs unblood Yautja, he would need some help...

And to answer the topic question, he did stab it a little near the back of the main part of the head, near the frill, but no, the queen still might have lived through a head shot...

~YOUR DESTRUCTION, IS THE WILL OF THE GODS, AND WE ARE THEIR INSTRUMENT~