This is almost by design, so you would spend runes to keep making progress. Once you go past this level you’d be cruising for another 10 levels or so, and then this will happen again. I’m on Mysterious Marshlands 50/56. My bp is 4910 and enemy roster is 4700. There are four waves and I can’t even make it through the first wave without losing 3-4 dragons of mine. I’m very frustrated and upset just like you.
Typically this happens with 3-4 waves of enemy foes, and with
AOE and healing power moves. Since they accumulate spirit faster than players, especially for those slower moves, they can be quite strong.
Stacking strong colors like @Talisax is saying could help, assuming you have multiple trained dragons of the same color, but the effectiveness of that is rather limited.
The easiest way out is to spend 100 runes to power through the level, or spend runes to draft stronger dragons and more runes to train and level them up, or keep grinding away at the repeatable tasks till the bp of your roster is significantly higher than that of the enemy foe. Sadly none of the three sound awfully inviting.