Lukas's plan was simple, at first anyway.
In essence, he wanted to confuse the ghast by constantly swapping himself and his doppelganger. To achieve this, he needed a few things to go right and a few things to work exactly as he needed them too.
To start, he wanted to cause the ghast to make a false assumption. He did this by switching his weapons as the false Lukas took his original place. Now, assuming the ghast wasn't more intelligent than the average ghast, it should have assumed the fake Lukas to be real, because it was the one holding the tachi and stood in the same place Lukas previously was in. At least that's what he hoped.
Of course, it had worked as he hoped it would.
Based on that, Lukas had preemptively informed Axel to have the illusionary Lukas always have the opposite weapon than he did. Thankfully it worked, otherwise that preemptive work would have been for naught.
