Hi Valentine,
I too use the flash forge dreamer and I have this problem as well. There are a couple of resolutions,
1 - Use the dual extruder so that you print your rafts and your supports with a dissolvable filament like PVA (Dissolves in water) Or HIPS (Dissolves in Limonene.)
2 - You can actually use a different slicer software and that will increase the quality of both your prints and the ease of getting your rafts and supports off. here is a link to a review on different slicer software;
(I have just given you the results. If you want to watch the individual reviews then you’ll have to watch part 1.)