The KI is very good for a game of that time. Consider that there's a random factor involved in every single battle which makes the calculations very complicated. The game is far more complex and than Battle Isle and its expansions. Especially the later levels with different enemy KI require a lot of thinking, the KI isn't perfect but definitely better than in BI1/moon. Battle isle is easier and smaller while Nectaris is more challenging.

KI means AI

This game becomes quite challenging to the point where one mismove and it will cost you the game, however I find myself unable to beat level 51 (romulus) I just don't see what the catch is here, it looks like it might be an even battle, that's until you notice that you have no means to repair and the enemy missile buggies force you to play aggressively which is impossible since there is no means to repair. I've tried to capture the HQ but it's simply too far away and there is no way to save any of the infantry on the left side of the map.

Any ideas? Don't want to skip a level

Level 51 (CHANEL) is the hardest level of the whole game. It's in fact impossible to save the infantries on the left side. Save the giant, you can use it for distraction now and then.
When I beat this map by capturing the enemy HQ with an infantry I'd lost almost all units.
You have to fight defensively (but not totally), survive long enough to allow an infantry from the east to travel west to the enemy HQ.
It's beatable without using save/reload but it's a difficult, slow and painful level.

Well I finally did it I saved one of the infantry at the west end (the one accompanying the giant), I used my buggies to secure his escape then waited for the right time to assault the HQ. My eastern force got annihilated, although they put up a valiant fight (even managed to take out lots of lynxs) but the three infantry there couldn't make it across the small canyon since the lynx buggies blocked them, so they were assigned to guard duty.

Well done :OK:
I didn't think that it's possible to save the infantry in the west. I didn't think the computer would leave the rabbit alone, I thought it would just tear the rabbit apart, encircle and destroy the giant and wipe out the infantry with its rabbits.

The next levels will be more fun.

You will definately lose one rabbit but its low price to pay to save this guy, I actually sent like 3 rabbits the computer player went nuts trying to kill them, but I retreated as soon as the infantry was safe.

Next level was way too easy though, I did fail in my first try since I wiped out the enemy of my ally leaving him without any enemies to fight so he won :P