Here's some tips regarding how to pwn everyone, and by everyone I mean EVERYONE, on Disc 1. I have personally dispatched Brothers, Seifer and Edea in only 1 Renzokuken.
Step 1 : Water magic
Availability : As soon as the game starts.
(Note : The monsters you need to defeat are quite harder than normal monsters in the Balamb area, be careful. )
Put Shiva to learn Ice/Water Refine and put Quetzalcoatl to learn Card Mod. Go to the beach area and fight Fastitocalon-F's until Shiva learns Ice/Water refine. Refine the fish fins that the fishes dropped - you'll recieve plenty of Water magic. 20 Waters per fish fin, to be exact! So make sure Squall and Quistis have 100 Waters each. Make Shiva learn Str-J and junction 100 Waters to Squall's STR. Make your way through the Fire Cavern. Make sure Squall's HP value is yellow (critical). When you get Squall's turn, you should have access to Renzokuken - if not, press Triangle until you have. Use Renzokuken. Rinse and repeat. Have Ifrit learn Str-J and junction 100 Waters to Quistis' STR. For Biggs, Wedge and Elvoret, Renzokuken away (MAKE SURE YOU DRAW SIREN FROM ELVORET!). For X-ATMwhatever, Renzokuken. (But if you want to kill him, junction 100 Waters to Selphie's MAG and have her use Slot and Do-Over until she has Thundaga a couple of times, or maybe even stronger magic *cough*meteor*cough*. I never tried it, but I think it'll work.)
Step 2 : Curaga magic and over 3000 HP.
Availability : As soon as you get Siren, but you'll probably need to buy some tents from Balamb/Timber/etc.
Have Siren learn Life/Recovery Refine and refine tents for Curagas. Junction them to your characters' HP. You'll have a buttload of HP.
Step 3 : -aga Magic
Availability : As soon as you get to Timber
Buy Magic, GF, Draw, or Item Scrolls from the Timber Pet Shop and use Tool Refine to make them into 10 Wizard Stones each. Refine them into -aga magic. You should have 100 of each elemental -aga spell on all your characters if you have the Gil, not necessary though. Junction them wherever you like.
Step 4 : Quake magic and 1 hit kills.
Availibility : As soon as you have access to the Tomb of the Unknown King. (Note : I don't know, but I think that you can win Armadodo cards from other places before TotUK.)
Unfortunately, you'll need Diabolos for this. To beat him, draw-cast Demi on him 2-3 times and proceed with your limit breaks. Namely Renzokuken, Zell's 2-move-loop (Booya/Punch Rush/Booya/Punch Rush/Booya etc.), Selphie's Slot with powerful magic, etc. etc. You'll beat him. Have him learn Time Refine. Make sure you have Quetzalcoatl's Card and Card Mod abilities. Use the Card commands on the Armadodos in the TotUK. Card Mod them into Dino Bones, and Time Refine them into QUAKE MAGIC!
Junction that to Squall's STR. Awesomesauce.
Optional step : STR Overkill
Make sure Ifrit has Str+40%. Challenge Zell's mother in Balamb to TT until you get Zell's card. Card Mod it into 3 Hyper Wrists. Use one on Ifrit. Junction Ifrit to Squall - make sure he has 100 Quakes junctioned to his STR and the Str+40% and Str+60% abilities on. Your Renzokuken should totally overkill everyone.
Optional step : VIT Overload
Win Gayla cards from TT players (I don't know what TT players have them, but heck, you'll find some). Refine them into Mystery Fluids, and Status Refine them into Meltdowns. Get 100 of them. Make sure Shiva has Vit-J and Vit+40%. Refine Minotaur card into 10 Adamantites and use one on Shiva. Junction Shiva GF along with 100 Meltdowns on their VIT and Vit+40% + Vit+60%. Did anyone say physical invincibillity? Well, at least in most of Disc 1...
Optional step : LIONHEART and 9999s flowing like water
So you got 100 quakes, a high level, some meltdowns to use on enemies, and you hit very hard... but how about a steady flow of 9999s? Yup, Lionheart on disc 1.
This video explains it all:

Want even more? Unfortunately, the only other things you can do to increase your damage is to use Meltdown on enemies for Vit 0 and to level up with Squall.
Good luck pwning everyone.
Step 1 : Water magic
Availability : As soon as the game starts.
(Note : The monsters you need to defeat are quite harder than normal monsters in the Balamb area, be careful. )
Put Shiva to learn Ice/Water Refine and put Quetzalcoatl to learn Card Mod. Go to the beach area and fight Fastitocalon-F's until Shiva learns Ice/Water refine. Refine the fish fins that the fishes dropped - you'll recieve plenty of Water magic. 20 Waters per fish fin, to be exact! So make sure Squall and Quistis have 100 Waters each. Make Shiva learn Str-J and junction 100 Waters to Squall's STR. Make your way through the Fire Cavern. Make sure Squall's HP value is yellow (critical). When you get Squall's turn, you should have access to Renzokuken - if not, press Triangle until you have. Use Renzokuken. Rinse and repeat. Have Ifrit learn Str-J and junction 100 Waters to Quistis' STR. For Biggs, Wedge and Elvoret, Renzokuken away (MAKE SURE YOU DRAW SIREN FROM ELVORET!). For X-ATMwhatever, Renzokuken. (But if you want to kill him, junction 100 Waters to Selphie's MAG and have her use Slot and Do-Over until she has Thundaga a couple of times, or maybe even stronger magic *cough*meteor*cough*. I never tried it, but I think it'll work.)
Step 2 : Curaga magic and over 3000 HP.
Availability : As soon as you get Siren, but you'll probably need to buy some tents from Balamb/Timber/etc.
Have Siren learn Life/Recovery Refine and refine tents for Curagas. Junction them to your characters' HP. You'll have a buttload of HP.
Step 3 : -aga Magic
Availability : As soon as you get to Timber
Buy Magic, GF, Draw, or Item Scrolls from the Timber Pet Shop and use Tool Refine to make them into 10 Wizard Stones each. Refine them into -aga magic. You should have 100 of each elemental -aga spell on all your characters if you have the Gil, not necessary though. Junction them wherever you like.
Step 4 : Quake magic and 1 hit kills.
Availibility : As soon as you have access to the Tomb of the Unknown King. (Note : I don't know, but I think that you can win Armadodo cards from other places before TotUK.)
Unfortunately, you'll need Diabolos for this. To beat him, draw-cast Demi on him 2-3 times and proceed with your limit breaks. Namely Renzokuken, Zell's 2-move-loop (Booya/Punch Rush/Booya/Punch Rush/Booya etc.), Selphie's Slot with powerful magic, etc. etc. You'll beat him. Have him learn Time Refine. Make sure you have Quetzalcoatl's Card and Card Mod abilities. Use the Card commands on the Armadodos in the TotUK. Card Mod them into Dino Bones, and Time Refine them into QUAKE MAGIC!

Optional step : STR Overkill
Make sure Ifrit has Str+40%. Challenge Zell's mother in Balamb to TT until you get Zell's card. Card Mod it into 3 Hyper Wrists. Use one on Ifrit. Junction Ifrit to Squall - make sure he has 100 Quakes junctioned to his STR and the Str+40% and Str+60% abilities on. Your Renzokuken should totally overkill everyone.
Optional step : VIT Overload
Win Gayla cards from TT players (I don't know what TT players have them, but heck, you'll find some). Refine them into Mystery Fluids, and Status Refine them into Meltdowns. Get 100 of them. Make sure Shiva has Vit-J and Vit+40%. Refine Minotaur card into 10 Adamantites and use one on Shiva. Junction Shiva GF along with 100 Meltdowns on their VIT and Vit+40% + Vit+60%. Did anyone say physical invincibillity? Well, at least in most of Disc 1...
Optional step : LIONHEART and 9999s flowing like water
So you got 100 quakes, a high level, some meltdowns to use on enemies, and you hit very hard... but how about a steady flow of 9999s? Yup, Lionheart on disc 1.
This video explains it all:

Want even more? Unfortunately, the only other things you can do to increase your damage is to use Meltdown on enemies for Vit 0 and to level up with Squall.
Good luck pwning everyone.
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