The problem is not that burning and poisoning do too much damage, the problem is that we encounter these things too early in the game.
No, unless you want "these things" to be removed from the early game.
The problems actually is in the fact that these elements do to much damage over to short of time - in TB combat and especially in real time, outside of combat.
As your own suggestion bellow actually confirms. Which is something every player knows as a fact very well.
Also damage is high and duration is short. Reducing the damage and increasing the length would give players some time to react and would make removal spells more useful.
As i was saying... and thats exactly what my suggestion would handle - through the mechanics of the game, instead of through some meta measures that would ruin the game.
Undead at level 2 or 3 should be skeletons doing physical damage. Having to deal with poison and burning at level 2 or 3 when you don't already have the response to it is a problem.
That doesnt make sense since you yourself can deal these same types of damage at those levels.
In case of undead - it is only archers who do this - and they have two special arrows at the most at those levels. Or mages - who have their spell lists intentionally neutered by the devs so they cannot damage players too much.
They are doing too much damage compared to low level health.
It is only the problem of players not even trying to use the right equipment with right kinds of resistance bonuses or to use any food and other items such as potions - that provide real resistances.
- when we are talking about combat against enemies.
When we are talking about environmental effects then resistances dont play any role at all. They have no effect whatsoever.
But if you reduce the damage, then these conditions will barely scratch a high level player.
High level players will never meat low level enemies.
You can scale the damage to the player's level ( not very realistic ) or introduce conditions later in the game.
what a horrible unimaginative idea.
... level scaling and just removing whole features....
Burn it with fire.