I used a standard strategy. Before the battle I set up Ramza as a Chemist with auto potion and the max amount of x-potions (this way whenever he is attacked Ramza will heal himself for 150hp) He was also equipped with the best gun I had and had "guts" as a secondary ability. During the duel with Wiegrahf I had Ramza get out of range and begin using "yell" to max out his speed. At this point he had had about four turns for every one Wiegrahf had. Ramza shot Wiegrahf mutiple times and defeated him easily. With Velius, Ramza should have about five turns in a row so fire away while your other characters hit him with everything they've got.