Theres plenty of "Cheap" ways to get around the fight with Wiegraf, some more cheap than the others.

Extremely Cheap method: Auto Potion on Ramza, destroy all your potions except X-Potions, Squire or Any other class with Guts secondary (Having your other skillset aside from Guts contain some method of healing can be nice in case your Brave level is too low to activate X-Potion after every attack. White Magic, Items, Monks Chakra, Samurais Murasame, etc.) Equipment doesnt matter, but as a squire you'll just want the best you can buy at this point (Whats that, Platinuum sword, germinas boots helps here).

Start the fight with Yell, and continue Yelling until you get 4 turns to each of his turn. Then begin Accumulating until your PA is over 30 (You want to be able to 1-shot Wiegraf,i.e. hit for over 500). If he Lighting Stabs you and you dont proc an X-potion, use your healing ability (items, magic, chakra, etc) to make sure he doesnt kill you next turn. Once you reach the aforementioned stats, run up to Wiegraf and 1 shot him. When he transforms, you'll still have 4-5 turns before he gets a chance to move since your stats carry over. Run up and destroy Velius before he can launch any attacks

The lesser cheap method: Keep the auto-x-potion, and use battle skill to break his stats down to 1 PA / 1 Speed. This will reach the same end result, except now when you fight Velius you arent vastly overpowered (in case you wanted to truly experience this fight without gimping Velius compared to Ramza)

Once you get to Velius, and assuming you didnt go super cheap method, prepare for a nasty fight. Your other characters need to be equipped with accessories to prevent his status effects. I cant remember offhand which ones he does, but you'll need to use stuff to make that negligable. If you cant prevent all his status effects, have 2 people capable of curing. Monks with Stigma Magic, White magic with Esuna, a mastered Chemist with Remedies, etc. Having a strong summoner in this fight helps to try and knock out his minions, run up and try to get a short-charged Bahamut on them before they can cast their spells.

Personally, I never give Velius a fighting chance. My first time through the game I reached Velius, saved my game, and ended up with no possible way I could see to beat him. My Ramza didnt have Auto Potion, my characters werent quite mastered enough, and without any time to train, I was stuck and had to restart. Ever since that first game, and I found out about auto potion with x-potions, I never give Wiegfra/Velius a chance to hit me