@redflame: it says "select 40 upgrades", what the dev actually means with "select" is "buy", you need to buy MORE than 40 upgrades to finish that challenge
@maestro1987: each level, as you know, comes with a level limitation on your towers, what it doesn't say is that the level limit also affects reinforcements, although I'm not sure exactly what the limit is for each specific level. I don't have the spear throw but I can tell there's a difference in my reinforcements hp and attack level in different maps.
@usher: If you're talking about the first boss on lvl5 (and not the 5th boss), you just have to answer to his movements: jump before he stomps the ground to avoid losing 20hp, walk back and slash the bubbles when he blows, when lance throws mines hit them and push them into him for 100 damage, kill the bushes he summons for a chance at extra mp or hp (also when you kill him, watch out for the 6 or so bushes that come out)
Try connecting to a smaller game with less players.