I'd say go looking up some battle themes from Darkest Dungeon. A few of those struck me as decent for a fight in DnD.
Terraria Community Forums
Techhunter_Talon
Matsu
Some of the tracks I use for boss fights are:
Battle with the Four Fiends covered by The Black Mages
Jecht Battle from Final Fantasy X
Boss Battle from Chrono Trigger
Vs Champion from Pokemon X/Y
Ganondorf Battle from The Legend of Zelda: The Wind Waker
Final Boss Theme from The Binding of Issac
Hiroari Shoots a Strange Bird from various Touhou games.
To name a few. If none of these work I can list more.
Battle with the Four Fiends covered by The Black Mages
Jecht Battle from Final Fantasy X
Boss Battle from Chrono Trigger
Vs Champion from Pokemon X/Y
Ganondorf Battle from The Legend of Zelda: The Wind Waker
Final Boss Theme from The Binding of Issac
Hiroari Shoots a Strange Bird from various Touhou games.
To name a few. If none of these work I can list more.