It's always hard to defeat The Elite 4. In Fire Red, you have to grind inorder to challenge the E4 (you can defeat the 8th Gym with a team of Lv 35-38 if played wisely. However the E4 uses all the Pokemons over the 50's Lvl).
Lorelei is one of the Hardest if you don't have a good counter to her Ice/Water type, specially Lapras. I always struggled against Lapras, as its damn Bulk & good offense puts holes in my team (since i usually don't carry a Fighting or Electric type).
Bruno, in my opinion, is the easiest of all. His Onix(s) gets killed by any Water type move and Hitmon's is really easy to kill. Machamp is the only member of his team which I fear to. But is still easy to win.
Agatha is the 2nd hardest of the E4. Her Haunter & Gengar are fast & Gengar probably 2HKO's any Pokemon my team carries. But my Shadow Ball Snorlax always did it for me.
Lance is ofcourse the hardest one. His Dragon's are not fast, but still has enough power to 2HKO every member of the team. I lost to him many times when i was new. I still lose sometimes without Lapras (it's the only Gen I Ice-type i love a lot).
But no one is like Blue. When i use Blastoise, i think it's too weak compared to other starters. But when he uses it, it's like the most powerful Pokemon in the game. idk why. I lost to it most of the time while facing the E4. Have to use lots of Full Restores & Revives to defeat him (& his Blastoise).