Hello,
back in the past I only played BI2.
Never the other ones.

So I started BI1.

Is it me, or this first series is quite difficult ?

I could do few maps, always y restrating few times and trying again, but here I'm on one which is quite difficult.
I can't tell which one though, I did not count. The game reports "29" but I'm sure I did not play 28 maps before.

I think it is the 3rd one with planes, the 1st one with transport planes.
The ennemy starts with like 4 fighters and 2 bombers, he quickly gets 4 copters and the player starts with 2 fighters.

Imarion

The game becomes harder as you progress but it's still beatable with some strategy.

Level 29 (the lower levels are player vs player maps) is SLAVE but it doesn't fit exactly fit to your description. Don't forget that you have a fighter inside your carrier and can use the blockers in your sea horse to your advantage.

Conquer the factory in the southeast as soon as possible.

The expansions are in fact harder than the original so I wouldn't bother with them before the original has been finished.

BI1 is simple.

- whenever possible do your first hit with ranged Units to weaken the Opponent and collect EXP
- if the opponents unit down to 1 or 2 take one of your Units with 6 life and collect the 2 EXP, this will make your unit better and most of the time you don't loose anything
- do not Forget to repair - high EXP Units first
- try to get Depots very fast, or leave them for later - on some maps you could even catch the opponents infantry...
- do not rush in - just make a good defence positon and let your Opponent attack - remember ranged attacks in 2nd line...
- you get more Score Points, when you kill all Opponent Units

now go an win!