Here are some ideas for gambits that could improve the dificulty and challenge
Gambit of the gravitation curse:
Blocks all movement spells cost when you have less than 100 mana
Gambit of the spiky road:
Every 100 blocks of distance that you travel with movement spells, get injured by 5% of your current hp
Gambit of the Magic disease:
When below 30 mana, you will constantly lose 5% of your HP per second
Gambit of the pity atacker:
Non-critical attacks will deal 50% of their original DMG
Gambit of the freezing instant:
Every 10 spells casted you will gain slowness 3 for 5 seconds.