The early game is always hard in pendor because there're plenty of units with stats way higher than anything in native. Over time your character will be on par with them and even better.
The various outlaw factions can be hard because of their knights: when they spawn with a bow they know how to use it. If you can't deal with them, train in ravenstern territory: the outlaws here will be accompanied by mystmountain units instead of knights which are a lot less dangerous (and overall give more renown).
Vanskerries deal tons of damage with their throwing weapons. It's a good idea to fight them alone if you don't have some pretty powerful units: grab a lance, run in circle until they're out of ammunitions and then simply play the lancer (it takes a long time but it's a good training).
Heretics are probably the easiest faction to solo (even early game with low stats): grab a weapon with decent range and speed and slowly run in circle around them killing the horses. It's mostly about you as a player here: with experience you'll be able to estimate the range to slowly kill them without taking a single point of damage.
Finally serpents can be dealth with roughly the same way you can deal with heretics. But (that's a big but) only if you can kill the priestesses at the very beginning of the fight: with their spears, long range weapons, crossbows and coursers they'll tear through your troops if left unchecked.
If you haven't done I'd suggest reading
One guide to kill them all; few things are outdated but it's still a great source of information.