For most situations you won't need to have many pieces of equipment to deal with immunities especially since most of the newer content lacks those. I'll answer as an RP - not having any elemental weapons available.
List of monsters immune to physical damage
https://tibia.fandom.com/wiki/Physical_Damage/Immune
As you can there are very few monsters that are fully immune to Physical damage outside of quests/events.
The most notable ones are:
- Ghost 
- Pirate Ghost 
- Phantasm 
- Burster Spectre 
And all of them take damage from the basic elements - with fire being the best one to use.
Usually when dealing with them running through/away is the most efficient way to survive. In other cases use bomb/wall runes and if you have to kill - for EK use an elemental weapon - for RP mas san and ava/gfb.
What's more important in my opinion is to have a versatile toolbox of secondary tools and items
Tools obtained in Secret Service Quest - https://tibia.fandom.com/wiki/Secret_Service_Quest
All of them have several tools included within ranging from rope and shovel to niche ones like a spoon
Obsidian Knife - https://tibia.fandom.com/wiki/Obsidian_Knife_Quest
Used in several quests, for skinning, in sculpting and more. https://www.tibiaqa.com/25676/what-are-all-of-the-uses-of-the-obsidian-knife
Replica weapons - especially wands/rods - https://tibia.fandom.com/wiki/Replicas
Invaluable when you meet a rare boss and don't want to instakill it - they deal 0-10 allowing people to share low health bosses like Willi Wasp and others
Blessed wooden stake - https://tibia.fandom.com/wiki/The_Blessed_Stake_Quest
Significantly less usable than Obsidian knife but still has uses in several quests
Melee weapon
Any melee will do but Knife is the best one in my opinion since it weighs very little.
Mandatory for destroying obstructions - spider webs and broken bamboo walls have to be destroyed in several quests
Music box
The only way go guarantee taming of some of the rare spawn mounts like Draptor or Midnight Panther
Temple Teleport Scroll
Invaluable when exploring. Can be used to instantly teleport to temple. Hate running back a forth to remote places in tibia? Just run once and tp back for free one a week.
Additionally on higher lvls this can be replace by The Supreme Cube
Runes
Destroy field
Has to be used in many quests but also some entrances between floors are obstructed by fire/poison fields
Wall/Bomb runes
Runes of all three available elements - fire, energy, poison - are invaluable as they can be used to trap monsters, section yourself from enemies and even damaging some bosses/monsters for bosstiary and bestiary
AoE runes
Avalanche, GFB, Thunderstorm, Stone Shower. Because sometimes you have to deal damage of a particular type
UH
Less usable for knights but still a must have in any backpack. Can be used to save others but also usable during several bosses - Yselda, Ravenous Hunger
SD
Best single target rune to have.
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