Denethor used Flare Blitz!
It's supereffective!
Brutal is exhausted!
...
Jokes apart, I'd like to give me opinion about Denethor's death. Personally, I don't like what you suggested for two reasons:
1. Personality: true, in the books and in the movies as well Denethor was driven mad by several things: Faramir's "death", Gandalf's "coup d'etat", Sauron attacking Minas Tirith, so he chose to burn. But, I don't think this means that in a different situation, Denethor would have liked to die in such a way. As Boromir says, Denethor was once a great and honorable man, crushed by the weight of responsibilities. Hence, being apparently not insane in a standard skirmish, I think that, being a great leader of Gondor, he would prefer to be killed while fighting the enemy and protecting his realm
2. Logic: how it is supposed to set himself on fire in the middle of the battlefield, as soon as he dies? He wouldn't have the time nor the necessary tools to do that, unless he's a demigod, or a Pokèmon (thus my initial joke
)