Another question about volcanic doomfire..?

If say there are 5 monsters on my opponents side, and they are in attack position. I then special summon a Volcanic doomfire by tributing 1 tri-blaze accelerator on my field. Does the monsters on my opponents side all attack my doomfire first and then the effect of doomfire kicks in with all monster destroys +500 inflict lifepoint per monster to opponent or i choose a monster to attack me then the effect kick in? im very confuse by this. Im also using the combo Ojama trio and battle mania. If u have any suggestion please write them down. Thank you. ^^