Scenario 1 is the easiest to describe, as there really isn't much going on. You cannot hire any mercenaries, so the only characters you control are Sigma and Lambda, whose positions are pre-defined. Rainforce and a Fighter can be found to your south-east, and another to your west, whilst to the north Aizel begins his attack. They will all immediately head towards you to attack, so you will have to act quickly - but you'll need a bit of luck to get the best out of the Scenario.

Let Lambda cast Fire on the south-eastern general as soon as you begin, and finish him off with Sigma. During turn 4, a clone called Omega will burst from one of the large crates, and attack Rainforce. This buys you some time, and it's best if Rainforce doesn't kill him immediately. Head off to the other Fighter and take him out, if you have time. Then head up to the doorway. If Sigma gets there first, you can tell lambda to hurry up.