Exposing the plant to increased amounts of UVB in the last couple weeks of flowering. Under a metal halide, UV-barrier glass gone! Outside? No way. THC is strain related for the most part and can be influenced only slightly to increase - the proper nutrient at the proper level at the exact right time. There are bulbs on Amazon (probably eBay too but I avoid them) with less barrier glass. Only the last 2 weeks, only MH, only with restrictive glass gone.