Imgor  was on the ground, too weak to get up. This was the second time this  day, which was definetly one of his worse days. He hopefully still was  able to do significant damage to the wyvern, or at least prevent it from  fighting back at all.
However,  he still wasn't done yet, even though he had next to no magic reserves,  and was weakened from a sudden drain of magic; his crossbow was loaded  and cocked, and when applied to a very very small radius for an instant,  he would still be able to cast his tracking spell using the shred of  magic he regenerated since the sudden drain, but even then, the radius  would be too small to reach the barrier from his current position.
Imgor  recalled the rune, that increased the range of vitae spells, if he  wanted to use his tracking spell to find out anything about the barrier,  he needed it to increase the range sufficiently to even reach the  barrier. He envisioned how it would look like when adjusted to he fairly  low range from his location to the barrier, and he started drawing it  onto the ground with his fingers, in the end, the rune would be roughly  hand-sized.

At  the same time, his familiar started returning to the ground below  Imgor, initially to provide shelter for him should the need arise. He  asked his familiar to draw a larger version of the rune under the ground  around him, not made up of tunnels, but carved into the tunnel's and  cave's floors or ceilings.
After  half a minute of drawing, the familiar's rune was finished, and  subsequently, Imgor cast his tracking spell, with the intent of finding  the weakest spot in the barrier, that has an effect on nearby parts of  the barrier; in essence a fault, that would cause it to at least  partially collapse, if said fault was damaged sufficiently. He hoped he would not pass out from using up all his magic for the 3rd time this day.

Interestingly enough his spell did not point out any particular point in the barrier itself, rather it seemed the wyvern itself was the weakness of the barrier. At least that's what Imgor managed to glimpse before his spell failed again due to low mana reserves.

A slight confusion set in in Imgor's mind, caused by a mix of the extremely low mana, the weakened body due to sudden mana drain after extremely low mana, and this revelation, so Imgor just stares at the pile of rubble, that trapped the wyvern.