Is there a limit to pet dragon level gain per battle?

[the benevolent adversary]

there is no hard xp cap but there is a soft one.
the xp you get for using your dragon is basically in relation to how many enemy are left and how many troops you have.
to find out better details search for it; there are many great threads here dealing with this issue.
800 xp is a great number my best was 1000 and some odd from a tough elenhall battle.
dragon dive seems to give the best xp for single use, while crushing blow gives the best xp to rage ratio.