In my experience(over 18 years in the fitness field), the body adapts to calorie deprivation and any further deprivation will result in a sluggish metabolism and lean tissue loss. The only effective and healthy alternative to contiunued caloric deprivation is alternating carb intake.
don't go lower than 8 times your bodyweight in terms of overall calories.
For example, Day 1- 40% of calories, Day 2-25%, Day 3- 15%-Day 4-35%. You get the idea. You can either vary the percentages or grams. Of course, your food choices matter here also. Try to stick to oats(but be careful they are allergenic), rice-my body thrives on thai brand rice noodles, sweet potatoes, and sprouted breads if you wish.