Many of the bn player go for level 70 but they don't want to do it just by shops, they want to do missions, contribute in events and fight bosses during the time they advance to level 70 (and continue when they are there). BTW: Heading for lvl 70 just running shops can be done in 5 months.
Unfortunately there are many different units needed to be kind of a good bn player - and even more not needed at all (just for collectors). I want to give some advise which units are good to spend ressources and time:
- Level 1: Trooper are widely underestimated. Use the chance to rank them as soon as possible - meaning don't fight with other units unless neccessary for doing missions (and give the the sp to the trooper).
- Level 12: The Wimp can dominate if you rank them to rank 6 early in the game. Unfortunately Laurels are needed to promote them - they can be bought for nanopod on low level.
- Level 13: Soldier (Team Fortress 2) are a unit useable up to level 70. Not many are needed in the beginning but they are very versatile and a very good anti-air on lower levels. The Mini Tank is the tanking unit until the Heavier Tank is available.
- Level 26: Buoy and Battleship Raptor-Class can be build. Or one can wait up to level 66 and get them pre-ranked at a higher rank.
- Level 31: SC-2 Pelican are the only plane available out of special events. They have good attacking capabilities against naval units and they can be used to protect other units when fighting critter.
- Level 35: Portable Wall help a lot, but not much are needed (unless you want to use them for heavy drops).
- Level 37: Frontier Chucker are good for giving drops when ranked up, indeed the attack capabilities are limited.
- Level 38: Riot Trucks are needed to complete some missions. Three are enough at rank 2.
- Level 42: Dragon are the first reasonable artillery but can't be used for a long time (higher levels). Unmanned Machine Gun Turret can help during the next 10 levels. As they need only gold making them is unlimited...
- Level 44: Light Chemical Tank are good anti-silverwolves. Not much are needed, too.
- Level 46: The Lightning Dragoon is a medicore unit but useful for heavy drops.
- Level 47: The Heavier Tank is the best non nano/promo tank at higher level.
- Level 48: The Cryo Trooper is the only non nano/promo unit with freezing capabilities. A few are needed and ranked up to rank 3.
- Level 53: Bigfoot are good as meat shield and are better in some cases than walls (very special). I you can infect them to become Mauler - one of the best anti-tank units in bn.
- Level 66: Portable Electric Fence are the best defense unit and the attack makes it even more useful.
- Level 69: The Artillery Plasma Cannon will replace all other artillery units and even some tanks. And it is mass-produceable.
- Level 70: Royal Spiderwasp Warrior has good attacks and defense from rank 1. Some Radio Tech are needed to complete some missions and fight bosses for non nano player.
With these units and some others needed for missions the whole game can be played. Winning PvP fights is possible, too. But in most cases these units have to be ranked immidiately when they are available. And don't be disappointed when leveling up and a favorite units is less usefull or completly useless.
There are many nano units helping in some cases and of course promo units. BN can be played without them and every player has to decide himself how much he spends on these units.