Whats the most effective way to cure very severe fungal nail infection?

Oral meds: To date has the highest success rate. You need to check liver enzymes prior o going on them. They are not "dangerous", which seems to me a misconception. Also, you want to get rid of any athletes foot you may have. Lasers are an option, but carry a price tag of 500-1200 bucks.

Nail fungus: Still the gold standard with 90 % success would be oral anti fungals , you will need to see your doctor for blood test before you start fungal treatment.

Have them evaluated.: There are different topical and oral antifungal medications available for the treatment of nail fungus. Some may opt to have the nail completely removed prior to treatment. Specialized lasers are also now available for treatment of fungal nail infections. Have your nails evaluated and get the appropriate treatment.