Diplomacy is the ONLY way this can end relatively safely without nukes. At some point, Russia will be exhausted, and cannot continue the invasion except by mass destruction. At this point, Ukraine should enter peace talks and formally hand over Crimea to Russia (something they already lost) in exchange for a cease fire, the dropping of sanctions, and Ukraine EU/NATO membership. That will protect them going forward, and allow Putin to skulk away with 'victory'. In reality, he will have been defeated and his economy crushed.