If we're talking about Scania, as an example...
Sell Ilbi (27atk) for 25M.
Buy 8 sets of Tobi (23atk) 1.5M each total 12M.
Buy 6atk WG with the rest.
Now you have 8 sets of stars with an effective atk of 29.
It is totally pointless to buy 2 Steely and a 2atk WG to make it up to 27. You then have 2 sets of stars. It's far far better to buy cheaper stars like Tobi and a better WG. It will end up having the same atk but you will have way more stars.