Run a Kar98K speed build and a fully modded G43 (No scope on either). Bolties get you more xp for infantry assault so use your speed bolt most of the time and switch to the G43 when you have to panic. Make sure you use the ribbon boosters too.
Also hunt snipers and use their rifles, very effective and doesn't cost you any money.
By using that build, three infantry assault boosters, and sniper hunting I got the 94k experience for the last two ribbons for my second STG the other day.
Needless to say veteran membership helps.