There are many ways to play Lich.
For siege is use undeads. 5 Melees, 4 Warlocks, 2-3 Spiders, 2-3 Gold Mummies and 7 Heroes. I can farm towers without downtimes and get any human tower empty with one siege (2200+ rares each), only goldtroops are Spiders and Mummies. My Temple is 180k+ but i have a buddy why is able to do the same with way lower temple.
For Arena there are different setups - for different enemys. Any Unit can be usefull or useless, depends on what you face. For me - undeads work pretty well because high rankings are mostly rakans.
For Dragons you can do full Warlocks (maybe 2 Spiders) and loan 2 deadeyes. You dont really need a frontline, spiderlings and heroes are enough for me.