The first thing I did was go through and hire every soul I could just so they could xp up. Gear suffered but I got through it by basically slaughtering the mid/advanced levels as quickly as possible.
Game+ basically upgrades that gear with skulls later. I debated on grinding for Evni but yeah... no.
In theory, every board is possible with merely the base six characters, there's two achievements based on that. Hero++ which is all levels without it is currently listed as under testing. Might not be doable. My guess is super-sheep's the problem there. That's an inane amount of DPS you need to lay into it with. You won't be playing with a lot of the gear but meh. You can easily speed-level by going into options and pushing xp gains to 300% so you're not grinding XP.
In theory you can also adjust the XML save game to up your scrap, but I'm not entirely sure where that blinking file is hiding. My guess is it's up on steam's cloud somewhere.
Interesting game though. In particular I like the way the different skills can synergy. For example, if you super-crit your bowman and super bleed the zerker with the ice-mage holding the enemies in range of the zerker, he can get 60% bleed going (much better than the 30% off the archer's choice) and your bowman can just lay into the targets. If you've got a knight down-stream of a dragon that's just overwhelmed by the # of enemies, drop a frost-mage in front, let the dragon do dragon-fire to 'free' them one at a time from the slow, and the knight can go to work.
Over-reliance on archers is far too expensive, actually, as typically are dragons before 18% Nom. The price for a zerker-path with a single ice-mage or two to keep them in the 'zone' is about twice as cheap for the DPS output, if you can funnel the enemy. One particular board this is a massive issue on is the water-board on new game+ called Gone Fishin'. Royal PITA that one on max-level. Took a bunch of tries to figure out how to get it to work so that I blew up the big phasing guys so they didn't pop their load of midgets right on top of Avra. You end up playing with the 'special options' for archers that are almost randomly placed and it works fine.
My only real complaint is the over-reliance on the knight as (reasonably) the only source of anti-armor, and no 'special control' to tell the knight to concentrate on things with armor. The archer can get some reasonable armor-punch but it's just not really that strong. Armor allows bots to ignore cold (slow for this game) and when you start going for the extreme challenges they get some significantly high armor ratings. There's a 'strongest creature in range' but that's not always the best choice. As an example, it'll take 4+ archers fully boosted to bust through a critter that a knight can take at boost 2 pretty easily. This reliance on the armor as a primary mechanic and not allowing for the special option seems... odd.
One thing I do really like however is the ability to try to customize the damage potential for a character, and you can re-stat pretty cheaply. Basically, your characters will level but they can't use all their abilities unless you boost them in a particular mission, and boosting goes up geometrically for each boost level, but the abilities opened are much stronger. However, you could kit out one or two of a type of character to be superman at merely boost 3, or you could simply invest in all their traits to turn them into constant +s for a character regardless of boost. One of my clerics, for example, is maxxed out on inspire (Boost 5) and hangs around the big DDs, usually either the archers or the zerkers. Another one ignores almost all the high-end skills and is a huge healer, with group-heal and aid getting the big boosts, some HPs, and he hangs out near the 'front line', which is usually where either your armor-breakers are or where your berserkers are hanging around doing as much AoE as possible on U-Turns. He goes up first on the cheap and eventualy the zeal cleric goes in for the big pushes closer to the end.
In general though if you like TD games and you like RPGs but don't have the time to really sink into a good 90+ hour epic yarn anymore, this is a great little fix and each mission plays out in 20-30 minutes at most. Remember to read Avra's journal, it's frickin' hysterical.