I was going over this with some Monarchs a while back and this is how I attempted to explain things, hopefully this will help...
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Okay, so the Battle Phase consists of the following steps:
1) Declare a monster that will attack
2) Target a monster on your opponent's side of the field to attack
3) Activate and resolve quick-play spell cards and/or traps. If both monsters are still eligible for battle (IE no Sakuretsu's,
Book of Moon's, defenders changed) then we proceed to the Damage Step.
4) Damage Step - Broken down into 4 substeps.
4a) If the targeted monster is face down, flip it into face up defense.
4b) Calculate and apply damage.
4c) Resolve any effects.
4d) Send
Appropriate monsters to the graveyard.
Now looking at this model, there are several little nuances where monster effects kick in.
In a classic
Man-Eater Bug scenario, its effect is triggered during 4a, but it won't activate and resolve until 4c.
In a
Mystic Swordsman LV2 example, its effect activates during 4a, before the monster is flipped face up.
In a
Drillroid example, its effect activates during 4b, effectively eliminating 4b altogether. The monster is simply classified as destroyed, though we still have to resolve any effects that were flipped up.
In a
Reflect Bounder example, its effect activates during 4b. Bounder will inflict its effect damage before battle damage applied to the
Appropriate.
Now think about Dark Ruler vs.
Spirit Reaper and see where those effects play in 8^D