I haven't played much with Jecht, but he's brutal since he can even block some HP attacks like Cloud's Braver, EX Death Almagest and CoD 0-Type Long Beam.
I love to play beat 'm ups with brutal AI, makes every battle that you manage to win even more satisfying ;) I haven't played a game with this brutal AI at the tougher level like since DBZ Budokai Tenkaichi in SSJ3 difficulty level xD.
Gabranth to me is the absolute beast in EX mode. With an EX Build and EX intake +8M you'll be almost always with a full EX bar, and his Innocence and Guilt HP attacks are the most brutal and punishing HP moves on the game, plus they have very few lag.
As for squall using only Blasting Zone as his HP attack, may I suggest you using Revolver Drive for ground and Rough Divide for Aerial? I found them to be Leon's best HP attacks.
You should try fighting in the Blackjack Duel Colosseum after you finish the Inward Chaos, if you think fights against the AI are brutal already on the Lunar Whale and on Inward Chaos, then Blackjack battles will make you scream at your PSP, especially those Maximum AI Level 130 xD
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
