>>636770451. No you can do a clear of 1-10 with just base units. Nousagi and Elfriend at lvl 5 and knight at 7 (just for extra HP, but not needed). Elfriend is the main DPS, and you need two big waves. On just before the boss and again after you lose the flag to get extra motivation. Just have to treat it as a boss battle and it's it can be completed.
2. Player engagement and returned logins. Can't defend it but that's unfortunately normal.
3. Higher rarety = Higher stats, there for higher cost. To balense the team between early round for cheap units to build up to the heavy hitters.
