Buy as much UST as you can with $1.5 Billion @ $0.15/UST. $1.5Billion / 0.15 = 10 Billion UST
Don’t burn bought UST instead sell it @ $1/UST. If people buy at this price you will have $10 Billion to back UST.
Restore LUNA-UST burning minting mechanism and people burn LUNA. Holders of LUNA burn LUNA for UST (@ market price) and LUNA supply will go down significantly.
Result: you have $10Billion USD to back UST and rest is backed LUNA/UST minting/burning mechanism.