If the monster battling
"Mythical Beast Cerberus" is removed from the field before (or during) damage calculation, the Spell Counters on
"Cerberus" will not be removed.
Removing the Spell Counters from
"Mythical Beast Cerberus" does not start a chain.
The Spell Counters on
"Mythical Beast Cerberus" are removed after he battles no matter which player is attacking.
You place a Spell Counter on this card when the Spell Card resolves, so if
"Magic Jammer" is used to negate the Spell Card's activation then you do not place a Spell Counter on this monster. But if
"Imperial Order" is chained to the Spell Card, you still place a Spell Counter on this monster because the activation of the Spell Card was not negated. If the Spell Card was a Continuous, Equip, or Field Spell Card, and
"Dust Tornado" is chained to its activation and destroys it before it resolves you still place a Spell Counter because the activation of the card was not negated.
"Mythical Beast Cerberus" must be face-up on the field when the Spell Card is activated in order to get a Spell Counter.