It should be enjoyable enough without understanding anything, you just shoot stuff. Storyline will probably be the same generic sci fi crap that Treasure do anyway, with a game like this it doesn't matter as long as the gameplay is fun. Pretty much like the first game.
In the TROOP database, select which enemy you wish to have join the fight and insert them into the scene like you would any other enemy. Right click them and select "appear halfway". Then down below in the events panel, set a conditional event for when your main enemy hits a specific %hp and select the "Enemy Appear..." command on page 3. Then select the enemy you wish to appear!