No, I wouldn’t really recommend JF as a titan killer. He’s not lame, but he doesn’t have any special powers there.
Elena, on the other hand, is unmatched in her basic tile damage. She’s good in titan fights.
Nonetheless, JF gets my vote. I recently max ascended him myself. I really enjoy watching squads melt. Remember, his burn damage goes up with troops, in case you didn’t know. Mine’s level 75, and he’s up to a little over 350 burn/2 turns with a level 10 mana troop. I really like the fire.
Elena looks kinda squishy. I’ve also never made much use of counterattackers, so there’s not much excitement from me there.
If you’re looking for well-rounded versatility, then I’d also look at tournaments. Elena could be awesome in Very Fast tournaments. JF is a master in Buff Boosters, though.
Now for war. Counterattackers are popular tanks for war. JF takes no damage from them when his special goes off. It’s very nice. Also, I found out heroes on defense get +20% to their Attack & Defense. It’s a little detail to round things out. But if I’m not mistaken, that means the players who fight my JF in wars have to withstand burn even higher than 350/2 turns. You’ll want to verify that, but it sounds painful.