In canon, the behemoth was known to be strong enough that it could not be killed by anyone other than Zald using what amounted to a suicidal charge to truly kill it.
I felt this act was only done AFTER they realized they weren't able to pull off defeating it without what basically amounts to a cheat. However, this also means that they thought they could defeat it in combat, without the cheat ability.
Since this is the case I imagine there were a few factors at play, but all of them would translate to a level's worth of difficulty. In my fan fic, this is laid out far more explicitly than the original series since I actually, um, wrote the battle…
So, now let's take the full might of the combined familia to see where they actually stack up. From everything before we know that collectively they are insanely strong. Reaching up to 1 Lvl 17 against non dragons. And double that against dragons.
Considering that Maxim and Empress thought that together they would be enough to defeat the Behemoth, we can make the observation that it is equal to about 1 Lvl 16 as a non dragon, or 1 Lvl 17 as a dragon.
During the fight, it became clear that Zald was absolutely essential to destroying the utterly massive monster.
Alfia's chant, while still just a level 16 attack, is still enough to cause massive damage in a limited area, even to a level 17 Monster.
So, the fact they knew they couldn't defeat it on their own any longer, means it was actually a bit stronger than Lvl 17, but also, not enough to the point Alfia's attacks wouldn't do massive damage.
Next, we need to consider the size of this monster which gives it the illusion of being stronger, when it really isn't. And then the environmental effect which is definitely a +1 to the rating level.
Finally, I'll divide the results up as follows:
Behemoth's Base : Level 16
Behemoth's Difficulty Rating: Level 17 < x < Lvl 18 Or in other words, Pseudo Lvl 18.