As ranged attacker Leo is quite helpful in the Dawn Brigade chapters when you need someone to soften up enemies for Nolan, Edward, or any DB member to kill. His defensive parameters aren't really an issue because he should not be attacked directly if you positioned him properly and Leo's main target of attackers are those who can't counter him back when he attacks them.Leo is not useful past maybe the first 2 levels. He can't double for anything, and his damage only about 4 to 8, which isn't the kind game breaker that Nolan or Edward can muster. His defense is also quite shoddy, if anyone needs babying its Leo.
You quitely underestimate what Rolf can do. Sure, Shinon comes at a higher level with better stats in most areas, but its not like its impossible to level Rolf, as he has much room to grow and surpass Shinon. Rolf can do significant damage to enemies and hold his own well enough. Besides, being at a lower level means he'll grow that much quicker, and use less BEXP then Shinon should you use it on him. Although, you shouldn't take anything I say seriously, as I am quite biased. Wolt=Rolf, yeah...
I don't feel I am underrating Rolf. "He's good when leveled up" I'm sure many units are good when they are leveled up, but Shinon doesn't need to be leveled up to contribute to the group when compared to Rolf.
Leo fills his role in the DB chapters and can be dropped later for the clearly superior Shinon, Rolf takes time to level up just to gain the same offensive might Shinon has to begin with. Well I have to give Rolf props for getting me a nice Silencer ^_^
Ilyana has the most chapters to level up when compared to any other mage unit (in fact I think she may have the most play time of all the units in the game). Thunder magic may be weak but it's not as if she's locked into only using that type of magic. With a transfered Ilyana her thunder magic level should be high enough to allow you to have Ilyana focus on other magics to increase her offensive might.Yeah I know Rolf is good in the game I just finished I had Rolf and Shinon as level 20 marksmen, Rolf gets higher attack faster but shinon reaches it anyways and he caps defense and everything else pretty much while Rolf is lacking a bit on the defensive, but There were both as even as you can get, still I have to hand it to shinon and since I never used leonardo, Shinon>Rolf>leo
One more question Llyana or Soren?
Ilyana also has Shade to somewhat help with her lackluster physical defense when not always properly positioned out of their range.
In my opinion Soren is the best mage but Ilyana is no slouch.