my 2 cents
Hit that crystal as hard as possible
The next cheap 5* ticket deal get some of those 3 hit untargeted types like bloomed lily rubellum, pitahaya, and wax tree (there may be others by then). They really help out the most when they hit that crystal! And a bracelet slot too if possible. No need for skill blooms since the are crystal fighters. Then Id maximize use of bloomed single target nuker types of which there are many of all colors. That's the cheapest way I can think to make a great whale team that will take that sucker down no problem! No need to fill it with rainbows.1.4 X 3 beats many of the rainbows 2.2 on the crystal
and once you got a sufficient power level any additional def/hp is wasted...you can focus purely on maximizing attack which is fun