Hadjane has a Grimoire, and in it are registered her minions.
To summon minions, souls are needed. These souls appear when enemies are defeated. Alternatively, tombs, found in various locations, are necessary.
Register the minions that appear from the various souls and tombs, summon them, and keep them at your beck and call, to use as hard as you wish.
There are many different devices that give Hadjane a hard time in the Underworld.
Conveyor belts, slippery floors, bursts of flame…
Approach them calmly, and make full use of the minions to get past them.
Rainbow Karma is used to purchase important items, and so on.
Rainbow Karma can be bought as an in-app purchase, but can also be gained as a reward when achievements are completed, or occasionally as a prize in the Board Game.