Two things - in normal mode it is generally very easy to win. First get to 15 level as fast as You can, then equip heroes that can reach dragon with equipment healing them on empty tiles, and finally make 15th level BIG, so they fight dragon only sometimes, rest of time wandering around and healing
Crawl mode - you regain HP while moving and not fighting, if low on HP just get to clear corridor and move back and forth - because of that it is impossible to lose crawl...