Some of the things I use to make monsters more lethal and dangerous, developed from my own ideas and from the suggestions you've made so far:
1. If possible keep players isolated from reliable or ideally any info on monsters
2. Play them as horror type creatures - use fear, stalking, incomplete exposure, slow build-up, show what monsters have done to others
3. Play them as Masters of Ambush
4. Use or develop entirely Unique Monsters with unique abilities
5. Use one-of-a-kind monsters or monster types that are very rare
6. Use rumors to give misinformation and disinformation
7. Rebuild popularly-known monsters so that they have unanticipated capabilities
8. Build chimeras of different monster types
9. Build Animal-Monster chimera types, monsters who possess real world animal capabilities, tactics, and defenses
10. Use enhanced monster sensory capabilities to full advantage (match the technological and magical advantages of the players against enhanced or even unique monster sensory capabilities)
11. Use monster intelligence to full advantage
12. Do not fight monsters to the death unless necessary
13. Use monsters instincts and survival capabilities to full advantage
14. Let monsters use their full advantages of initiative
15. Use monsters natural terrain and "home-field" advantages
16. Play intelligent monsters as fully developed NPCs with their own set of tactics, motivations, alliances, and capabilities
17. Use monsters whose capabilities change or mature or develop over time
18. Use monsters whose capabilities actually and really change into totally different capabilities over time or due to some sudden, dramatic adaptation or event
19. Use monsters as long-term and strategic threats, not just short term and tactical threats
20. Use monsters to threaten loved-ones, attack and kill comrades, and as creatures apt to take vengeance
21. Make full use of monster Special Attacks and Abilities: such as magic or psionic capabilities, etc.
22. Use monsters who set traps
23. Use the persuasive and negotiation capabilities of intelligent monsters
24. Use monster's previous experiences so that they adapt, mature, develop, and become ever more dangerous each time they survive and encounter
25. Intelligent monsters may use hirelings, servants, or have defenders, and networks loyal to them
26. Use monster preparations and defense lines/defense networks to their advantage
27. Let monsters hunt, and hunt very effectively, rather than just being hunted - do not make monsters static and emplaced, but give them open ranges of hunt, maneuver, motion, and action
28. Establish monster territories where they are far more effective than normal
29. Let some monsters have spy networks so that they can employ pre-emptive action
30. Some monsters should possess and be able to employ their own versions of technology
31. Use unknown monsters from other games and gaming systems
32. Use self-created monsters
33. Use monsters from myth, legend, etc. rather than typical gaming monsters
34. Monsters that infect, corrupt, or reduce their victims
35. Use monsters with supernatural and preternatural capabilities, such as the ability to foresee and foretell the future, sense creatures from other dimensions, etc.
36. Use monsters that can employ prophesy, can curse, or bless, or in some other way alter fates or destinies.
37. Use monsters with powerful body parts, or components whose magic or influence can survive death or affect things independently.