From what I read, using Neltharion's power would actually put her at a disadvatage. Ethereal Mode attacks are highly destructive, but purely magical in nature, and Ion Cannon Breaker has a long casting timer (hence the increase in defense in Ethereal Mode) all in all very similar to Nanoha's standard combat routine (big attacks, make things go boom) so unless Neltharion's corrupted energies can 'burn' through the Aether Threads (overpowering them, in a way, but this is up to An Hero) we're looking at a similar situation as Serge's battle versus Nanoha. Not to mention that Neltharion's power is actually dangerous to use for a prolonged time. It's like acid, you can dilute it to reduce its negative effect, but you can't get rid of it completely.
On the other hand, Dragon Soul and Seismic Charge have low casting timers, but are mainly spells destructive on a large scale, which would make it hard for Serge to manipulate the spell. For example, Seismic Charge looks like a red version of
this weapon (around the 00:40 mark and one more at the 10:00 mark, simply replace the bomb with a red blast and picture the explosion red. Keep the cool sound
). If it explodes, I don't think Serge can manipulate the entire wave.
But here is Keroko's biggest advantage: In a 1 on 1 battle, she doesn't 'plan', she acts (and from what I read, Serge can read
planned attacks). Most of the times her body acts before her mind even realizes what it is doing, catching up miliseconds
after. This was what gave her a huge advantage during the Keroko/Tesla fights, as by all means Tesla was a superior oponent. It's also a near essential skill for a dog-fighter. So while Serge might hold the advantage early in the battle, if he allows it to drag on, Keroko will not plan, she will react to Serge's moves and do so with nigh-impossible speed. It's a form of battle sense, you could say.
If you've got questions about specific attacks or elements, though, by all means ask.