How do i defeat nightmare general in slay the dragon?

[Fictioner]

It has high hp and super high dmg, cant seem to find a way to kill it/ get past it to get to fiesty dragon lair

[GronkJuice]

you have two options.

1) overwhelming power. (Protoss that ♥♥♥♥)
At about day 30ish you should have amassed enough of an army to face roll them.

2) kite
this take either well use micro or horses. First get him to target unit(s) to kite, next you take ranged units (as many as you can muster for DPS) and target him. Lastly you take those kiting units and run into and away from him when he attacks. minimizing damage. you can also have healers restoring damage.

Healers are key in more heavy fights. even if you want to face roll siege healers with a high HP front line, and targeting range can melt them.

[Sezostris]

I guess it depends on difficulty You are playing at, i finished the game on lowest so Your case may be different. You can nuke him down quickly with lots of balistas and crossbowmans and couple knights/mounted knights for tanking. Keep them alive with healing spell (preferably upgraded). With big enough back line You will kill him before any of Your tanks die with 3-4 healing spell casts. I didnt mention healers because in my experience General is just dealing way too much AoE damage for them to keep up, better invest those resources into as much damage as possible to keep the whole fight shorter.