Nidhogg is insanely overpowered when used right.You can just use his breath attack and then keep getting hit to get more energy.The breath attack heals you faster than you get damaged,therefore unlimited health and energy.
Easiest way to get sparta badge.
1.Buy grenade launcher
2.Upgrade launcher to full.
3.Buy quake boxes in turrets menu.
4.Upgrade boxes to full.
5.Spend the rest of your money on walls and repairmen (you probably wont need them tho.I didnt get hit once.).
6.Start.
7.Let them dine in hell.
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