Farm

• Training : Defence 20, Health 20, Power 20, Special 20 (use controller pause button and mash your mouse + spacebar at the same time for optimal speed, keep training for ~16 more seconds to get additional coins), Speed 20 (do the 1s then spam every key as soon as it starts giving you 2s and do that again until you get to level 20)
• Buy Boomerang in the shop and do the 3 battles.

Forest

• Training : Health 24, Power 40, Special 40, Speed 40
• Buy Pie in the shop and do the tourney, upgrade Pie to lv2 before the first guy (you can upgrade weapons and armor during the loading screen).

Mountain

• Training : Defence 60, Power 60
• Buy Shovel, Rain Coat and Rain Hat in the shop and do the tourney (don't upgrade anything).

Desert

• Training : Defence 80, Health 80, Power 80 (you can pause buffer to stall the enemy ducks but it's not required if you're good enough), Speed 80
• Fight the blue duck on the way to the shop so you can get Watergun
• Do the tourney, upgrade Watergun to lv4 before the first guy.

Swamp

• Training : Defence 100, Health 100 (just hold right to get XP)
• Do the tourney, upgrade Watergun to lv5 before the first guy.

City

• Training (Grand Dojo) : Desert Power 100 (Farm Power may also be viable, I should probably do more testing), Farm Special 100, Farm Speed 100 (same strat as in the farm)
• Buy Laser Gun in the shop and do the tourney, upgrade Laser Gun to lv3 before the first guy.