Kingdom Rush Frontiers (Steam) Impossible Hero Challenge - Storm Atoll Campaign

The game tries to ratchet up the difficulty here by playing with the geography.  We now have islands with build points that can't be reached by non-flying heroes.  There are also two exit lanes, and both of them are pretty cramped for space to defend them.

Fortunately (or unfortunately for a challenge whose purpose is to rank heroes) a good build strategy takes care of everything.  A Necromancer plus Mecha is all that's needed to defend the primary exit for most of the level, and a barracks and Mecha adequately defend the other with some properly timed Rain of Fire support.  Later in the maps we start building out Assassin's Guilds to deal with the Bloodshells.  When Sneak Attack is maxed out, it basically works out that every attack has a 1% insta-kill chance.  That might not sound like a reliable strategy, but with Counterattack maxxed out as well a single Assassin's Guild can stall a Bloodshell just about indefinitely (perhaps with a little reinforcement support to hold it in place while waiting for respawns).  It doesn't take all that long between three of them to get 100 attacks in, which does not guarantee an insta-kill will happen, but the odds are definitely in favor of it.

And when the insta-kill just doesn't want to proc, we have the Shrine of Regnos to fall back on.  We start upgrading it fairly early on, as we can use it to minimize some of the early threats from Bloodshells and Blacksurges.  We take advantage of the fact that when you upgrade the shrine the ability immediately becomes available.  Exploiting this we can use the shrine twice in quick succession two separate times - we upgrade the shrine to level 1, then when a threat appears we use it, upgrade to level 2, and immediately use it again.  We can do this again when going from level 2 to 3, and between this and Rain of Fire we can take out a lot of big mobs.

So there's not much for our heroes to do here except provide support at the main chokepoint, and everyone does that adequately well.  So no grades here, but perhaps we'll get some in heroic or iron mode.

Kingdom Rush Frontiers Impossible Hero Challenge - Storm Atoll Campaign