Because we make a snowman with heaping up a lot of snow,we say it is"堆雪人"dui xue ren."堆"means heaping up.That is to say,put a lot of things together and it looks like a hill. For example,"The gardener began to heap up the fallen leaves","A little pile of equipment began to heap up on the table". when we have a snowball fight,we say it is "打雪仗"."打仗da zhang" has several meanings.Sometimes we refer to the wars between armies.For example,we say "德国(Germany)和法国(France)打仗 de guo he fa guo da zhang".That means there is a war between the Germany and the France.Sometimes we refer to the brawling.It usually occurs between several people. When we have a snowball fight,we throw the snowballs at other people and maybe hit them.It looks like a brawling.But since our "weapons" are snowballs, we use the "雪xue"(snow) to describe the fight.We say it is "打雪仗". And just like that,if a war between armies is very tough and hard to win,we say it is 打硬仗 da ying zhang。“硬”means hard and tough under the circumstance. hoping it works.

when build snowman we say 堆雪人(dui xue ren) when fight with snow we say 打雪仗（da xue zhang）