Fighting big boys is often a matter of endurance, which means burst damage builds like nightblades or sorcerors may find themselves in trouble, as their usual "burn them down before they can do much harm" tactics won't work as well.
Definitely add some defensive and self-heal skills there, and watch your resources. And definitely watch for their special moves, block/interrupt/dodge as needed - with some of the big critters, all it takes is missing one defense to be in trouble...
One trick - if you learn to circle them -while- attacking, that will make things a bit easier...

Groups... three is about right for your average player. Some classes can easily do more (and I mean, without getting them all in one place and spamming AoE), but until you master your class, anything more then three will be difficult at first. And even when you have gotten good you may have to work for it if you face, say five wasps or scorpions in upper craglorn... but it will get easier
BtW, situational awareness helps avoiding "aggro cascades" (when you are fighting one group, the fight moves just close enough to a second that they feel inclined to join the fun...) Sometimes when a ranged enemy moves back for a better shot, its better not to pursue but find some cover to break LoS, thus luring them away from that other group again...

And yes, once you finish the main story, you get to do the other factions in cadwells. So you can do all the game content with only one character if you so choose. Or make a bunch, in case you find friends you want to PvP with on that side as well... as you like, and have time for.