Basically, the cactus missile barrages also trigger thorns. So the simplest method to get the thorns kills you need is:

1. Toggle off all tomes besides Thorns tome and Armor tome, Toggle off all weapons besides melee weapons

2. Choose Athena, Desert Map Tier 1

3. Pick up ONLY melee weapons, and the Thorns tome (mandatory), Armor tome (optional), plus any max HP or regen tomes/items. Level up these tomes as much as you can. Do not pick up items that kill enemies at range (cannon, soul reaper, etc.)

4. Wait for cactus to spawn. They can spawn at any time.

5. When cactus spawn, group them up while being careful not to kill them. Then, do some bhops/slides to get far away from them and stand still so that the missiles hit you. The missiles will trigger thorns damage, if your thorns tome is around level 4 it will oneshot the cactus.

6. Move around to let your HP regenerate, then stand still again and let the missiles hit you some more.

7. There you go, easy cactus item.

This method is by far the easiest and quickest, it still requires some RNG (cactus might not spawn at all in a run) but it's way better than trying to run into them with shotgun and sniper.