Hellkite Charger cannot be blocked and killed by an opponent.
Steps
Declare
Hellkite Charger as an attacker, triggering it and
Dawnsire.
Resolve the
Dawnsire trigger, attempting to deal 100 damage to
Crystalline Crawler, which will result in 100 +1/+1 counters being placed on it instead due to
Vigor.
Holding priority, activate
Crystalline Crawler's first ability seven times by removing seven +1/+1 counters from it, adding ({5} magic symbol) ({R} magic symbol) ({R} magic symbol) .
Resolve the
Hellkite Charger trigger, causing you to pay ({5} magic symbol) ({R} magic symbol) ({R} magic symbol) to untap all attacking creatures you control and to get an additional combat phase after this one.
Repeat each combat phase.
Notes
As long as
Crystalline Crawler has enough +1/+1 counters on it, you do not need to target it in step 1, allowing you to put +1/+1 counters on other creatures you control if desired.