Looking around on the GuideScroll Warframe guides I have found the following information:
GRINEER: Takes bonus damage from Armor Piercing.
CORPUS: Takes bonus damage and gets staggered by Electricity.
INFESTED: Takes bonus damage and some gets staggered by Fire, some are immune to Electricity.
Freeze damage dose 200% damage to shields where all other damge type do 100% damage.
Most enemies will recive a bonus resist based on there level that is subtracted from your damage.
Damage is listed as Normal hit/Weakpoint hit, for most that is Body/Head.
GRINEER
Everything is armored except for melee Grineer.
Normals include trooper, Lancer, etc.
Heavy include Bombadier and Gunner.
Shield Grineer takes damage as Normals, but recives 2 levels less resistance.
Commander-normal is the same as Heavy but immune to electrical/fire damage, AP/freeze not tested.
Normal Armor Piercing Fire Freeze Electrical
Sawmen 100%/300% 100%/200% 100%/200% 100%/200% 100%/200%
Normals 33%/100% 150%/300% 37.5%/75% 16.5%/33% 20%/40%
Heavy 66%/200% 100%/200% 50%/100%+ 50%/100% 50%/100%
CORPUS
Crewmen are armored.
They are Staggered by Electricity.
Moa's weak point is there midsection/gun.
Normal Armor Piercing Fire Freeze Electrical
Crewmen 81%/0% 100%/2000% 33%/100% 75%/200% 160%/400%
Moa 100%/200% 50%/100% 50%/100% 50%/100% 200%/400%
Osprey 100% 100% 100% 100% 200%
Cameras 100% 100% 100% 100% 200%
INFESTED
Normals include Chargers, Leapers, Runners, etc.
Ancient heads/bodies and Crawler bodies are armored.
Normals and Ancients are Staggered by Fire.
Crawlers have 3 areas of damage Body/Legs and Arms/Head.
Ancients have 3 areas of damage Body/Head/Lower legs and Arms.
Normal Armor Piercing Fire Freeze Electrical
Normals 100%/200% 50%/100% 200%/400% 100%/200% 50%/100%
Nauseous Crawler 66%/100%/200% 50%/50%/100% 125%/200%/400% 50%/100%/200% 50%/50%/100%
Crawler 50%/100%/200% 50%/50%/100% 100%/200%/400% 50%/100%/200% 50%/50%/100%
Ancients 25%/75%/100% 100%/200%/100% 37.5%/75%/200% 25%/50%/100% 0%
Channeling Efficiency reduces the cost per hit while channeling (holding left mouse button while having the sword equipped).
Base cost is 5 energy per hit (misses do not use energy), minimum energy cost is set to 1 Energy per strike, so even if you should get 100% channelig Efficiency, you can't get below that cap.
Also not that you can gain a damage multiplier while chanelling - ever 5 consecutive hits earns you a damage multiplier of 0.5 (I don't know if there is a cap).
Source: Warframe Wikia
Warframe forum
Best Answer
The level of the mission dictates the levels of enemies (or the levels they start at for endless defence). Bosses and nightmare mode's enemies are higher level than the mission level, but they also depend on it. Bosses in lower level missions will be much lower level than those in Pluto/Europa.
Difficulty seems to refer to the amount of enemies.