Got the impossible badge with 8 lightning and 3 shields. There were plenty of times where I was close to beating floor 14 with 6 lightning and 4-6 shields, but seems like investing in more lightning was optimal. Don't bother buying any splash. It can actually hurt your chances of beating floor 14.
Pro Tip on Gold and Exp Farming: Focus spending gold on the top row of upgrades. Use extras on the third row for "Battle Experience". Once you reach second lieutenant, buy "Fast Start". This will help you earn gold and exp really quick. Crash as much wooden barriers and enemy soldiers as you can with the jeep. Once your jeep is destroyed, suicide as fast as you can.
Thanks, we will check it.