In this Level there is a lot more room for error, then in the previous ones. Target Score is 30000000.
The Brutal build up is pretty much automatic when using LT with Ash.
To get into Focus, use Major Ruin - hold LT + RT. Connect and chain Hits with Ash (LT) to maintain/recover Focus and get Score per Hits/Kills, multiplied by x (Brutal), x (Carnage), x (Frenzy).
For every ten (combined) Multipliers, one limbo Troll appears. Try to only/mostly connect Hits to them, for as long as they are still available. These stop appearing after a certain threshold, but keep using Focus to build up Multipliers and Score nonetheless. It is a lot of fun too.
I got 80000000 on my first real attempt. That is very easy there, once you know what you are doing.
For this Level, I even recommend to start Focusing once you have a mere x10 Brutal Multiplier.
Always erase Fire Enemies first while keep moving and aiming/spraying Water. Also eliminate Sniper Trolls of any kind - it gives you peace of mind. Do not worry about getting hit sometimes. I think I got hit about 15 times or so.
Do not attack the Walrus - it is an Ally. Protect the Walrus at all costs. Ice Bugs are best eradicated with Ash standing on the middle of the Bridges in both defense phases - breath fire, of course.
With Focus, it is very easy to evade the Ice Spikes of the prime Enemy there. Do not worry, you will not have to battle against it there at.