If you cast a spell with casualty this way, you can pay the costs in any order, which means that you may remove a counter from a creature then sacrifice it to pay the casualty cost. Bear in mind that removing a +1/+1 counter this way will reduce a creature's power, which might prevent you from paying the casualty cost with that creature.

"Shield" is not an ability that creatures have and shield counters are not keyword counters. If a creature with a shield counter loses its abilities, the shield counter will still protect it as normal.

Removing a counter from a creature is an additional cost of casting the spell and may be combined with other additional costs or alternative costs that spell may have, such as casualty.

If a permanent that would be dealt damage has more than one shield counter on it, that damage is prevented and only one shield counter is removed.

Shield counters don't prevent players from sacrificing creatures.

A creature with a shield counter on it may still be destroyed by state-based actions if it has damage marked on it equal to its toughness or has been dealt unpreventable damage by a source with deathtouch.

Removing a shield counter in this way isn't the same as regenerating a creature.

If a permanent with a shield counter is dealt unpreventable damage, that damage will be dealt and a shield counter will still be removed.