@with

To say "kill WITH arrows", just add third ending to the word meaning arrows —

haniSye zaraiH हनिष्ये शरैः

To say "go WITH someone", in theory you might just add third ending too —

putreNa gacchati पुत्रेण गच्छति "goes with son", she goes together with her son

But in practice, more often than not people put the word in third AND add some indeclinable that means "with", such as **saha सह —

putreNa saha gacchati पुत्रेण सह गच्छति "goes with son"

This is clearer, because a sentence like —

ahiH putreNa hataH अहिः पुत्रेण हतः

may in good grammar either mean "with son" ("the snake was killed along with its son") or "by son" ("(my) son killed the snake"). Adding **saha सह forces the "with" meaning —

ahiH putreNa saha hataH अहिः पुत्रेण सह हतः "the snake was killed with his son"

We can also say —

ahis saputro hataH अहिः सपुत्रो हतः "the snake was killed with his son"

ahiH putraz ca hatau अहिः पुत्रश्च हतौ "the snake and his son were killed"

and that's why ahiH putreNa hataH अहिः पुत्रेण हतः will almost always mean "(my / your / his) son killed the snake".