Level 16: On normal, you face a super-angel. It behaves the same as your angel does, but it has more health and damage. It is immune to arrow damage. I recommend you let your melee titans get in range then freeze time while the angel is in the middle of an attack. Your melee units will then continue up to the castle and destroy that while you keep time frozen.
Oh hard, you face a black Lord of Fire. It is immune to arrow damage. It has A LOT of health, but it only attacks if a unit gets too close to it. Popo, cannons, the angel, sylphs, and succubi are some of the units that can attack outside it's range, I just didn't bother to try all units. Just make sure you order them to the spot that they can attack from rather than telling them to march, otherwise they will inch closer to the boss with every attack. The boss will move a few times, but never past his half of the field. Melee units are NOT recommended for this boss.
Level 13: Goblins are thrown all across the feild, causing your units to advance slower as they stop to kill them. Red goblins have extra health and absorb explosive damage. Black goblins can kill most of a fully upgraded titan's health in one hit. You have to use nonelemental damage to kill them. The dragon and angel have nonelemental damage, but they move slowly and don't always attack the goblins in front of them. It is better to use a meteor spell. Levels 1 + 2 of this spell are too weak to kill the goblin in one hit, but you can use berserk or rage just before using meteor to kill with one hit. On hard difficulty, the black goblin respawns. On epic difficulty, black goblins will spawn at about the same rate as a titan. The last revenge wave one epic difficulty will spawn many black goblins at once, so be ready. This is one of the best levels to get gold from. I've gotten over 1.6 million gold in one match on epic, and over 200K on hard (from all the goblins killed during game).
Level 14: There will be a large bomb that moves slowly across the battle field. This bomb does not attack, and will not stop for anything other than time freeze. You will likely need to use time freeze just to slow it down enough to kill it. When it dies, it creates a large explosion that no unit in the blast radius can survive. If it dies too close to your castle, your castle will die as well. To protect your units from the blast, you can either use shield before it blows, or freeze time and order your to units retreat. On hard difficulty, the bomb will respawn shortly after being killed. On epic, there will be a continuous spawn of bombs at even intervals. On the epic difficulty, titans will be pretty much useless. They are simply too slow to move from one bomb's explosion to the one behind it. Use fast units and have them move outside blast radius before it goes off. The bombs have a high bounty yield, making them good targets for farming.
Level 15: Small rocks fall on random parts of the field and cause minor explosive damage to any unit near hit area. Baal is near the middle of the map. He is just inside of your arrow tower's attack range. You will need shield or time freeze to get near him. Lord of Fire is recomended here. Once there, you can attack him without being hit by his sword (minimum attack range). On normal difficulty, he will absorb (be healed from) explosive attacks. On hard, he is vulnerable to explosives but immune to melee and arrow attack and the revenge wave will summon a Lord of fire. On epic, he absorbs melee and arrow attacks, is vulnerable to explosive, and the revenge waves will each have a Lord of Fire.
The only junk items or obsolete guns you will ever need:
2 broken pipes, 1 paper weight, 2 bags of nails, 1 roll of tape, 1 broken board, 1 ppm 9 pistol, 1 welding rod, 1 metal sheeting. Throw away everything else. The best weapon you can get is a katana. It can either be found or bought at the second store. You cannot sell items so don't bother holding on to old weapons if you have better ones. Meele is the way to go. Until you get a katana, use blunt items as primary weapon. You'll be using a nail board most of the time. Give your best gun to your partner and use your second best gun as your secondary weapon.
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Good catch! Don't know how I missed that :p