(Note: Was lv 10 Gill when achieveing)
1) Get: Gill Grunt and upgrade along the water hose path and fully upgrade to obtain simply put, the infinite jetpack/hose.
2) Method: First two rounds, easy food using gill. The minions will be standing in front of whatshisface, you'll easily get the food and damage whatshisface alot.
3) The hydras: Alot of people had problems with this, so did I... till I worked it out.
Life hydra: If you get damaged by this one.... I'm sorry, stop reading and give up now. Easiest one, pray for this. Slow moving attack, you'll well damage whatshisface before they're even close.
Water hydra: Stay CLOSE to the shark spawns. I know that sounds stupid but, it's really not, when spawning the sharks are widespread and easily navigated, instead of converged wayyy at the back of the arena.
Undead hydra: North-west corner of the arena, stand there, can't hit you, don't even have to move.
Fire hydra: South-east/west corners, same deal as undead.
From all these locations you can easily hit the whatshisface in the process.
Minions: I honestly, barely attack them. I spam pump at the start (without jetpack on) then face left/right, activate jetpack (so you continue to face that direction and lap in circles the arena, even the rangers round, they can't hit you, spray when your on the appropriate angle, don't leave the circular motion, don't even TRY to shoot whatshisface, unless it's guarenteed.
This sounds long and shifty but, by playing it safe and doing most of the damage during the hydra defense rounds, your life will be easier.
Honestly, I camped southern corners to avoid undead and fire hydra, nature can't hit you from that far, and you'll NEVER have sharks at the same time as undead, so don't worry.
Small tips: ALWAYS keep jetpack on except for the brief period before, jetpack increases your speed, you'll need it.
NEVER charge at minions. EVER.
Preferably, run anti-clockwise. (Personal choice but, I always tend to have less problems.)
Hydra's quickly switch, the tornado's on the left and right of the head will show what to expect before the head's change, sounds pointless but, if your a bit stressed/panicky, gives you those few extra recuperation seconds.
-Thanks, hope it helps.