Activate Anzrag by paying ({3} magic symbol) ({R} magic symbol) ({R} magic symbol) ({G} magic symbol) ({G} magic symbol) , causing opponents to block Anzrag if able this turn.
Cast Arachnogenesis by paying ({2} magic symbol) ({G} magic symbol) , preventing all damage being done this turn by non-Spider creatures.
Declare Anzrag as an attacker, attacking an opponent who controls a creature but does not control a Spider.
The opponent is forced to block Anzrag, triggering Anzrag, untapping all creatures you control and causing you to get an additional combat phase after this one.
Any combat damage that would be dealt is prevented due to Arachnogenesis.