The best advice I can give is to look at how your favorite fiction writers telegraph these cues to the reader, and try to do the same. Use foreshadowing, metaphors, and careful word choices to convey the message subtly. It takes practice but the payoff is pretty fun.
So the boss monster is vulnerable to slashing damage? Describe the wind as "cutting through you like a knife" on the way to the dungeon. Talk about how the merchant is "slashing his prices," and describe the bard's vicious mockery as a "cutting insult," etc. Occasionally describe slash marks on walls and doors, and place boobytraps that deal slashing damage. And so forth. When the time comes, everyone will have "slashing" on the brain but won't know why.