I thought that divine armor does not add its resistance to basic resistance of a unit.
That is, if a unit has greater resistance than given by spell (like 80% fire res of red dragons) it will gain nothing. (only other resistances)
I will try this strategy later on.
Now I am clearing other islands to get 8000 leadership and change my current 2 red dragons into 4 red dragons
Lucky I already got sacrifice spell.
I am also thinking about taking some useless units, let them get killed and kill half of enemy stacks with evlin spell...
Thanks for the advice!