Oh my gosh, I freaking did it!!! Okay so, here's the strategy I used. Unlock Diamond (for the high HP) and the crossbow. First order of business is to get yourself leveled up like crazy. Make a big circle around the enemies and shoot to the center so they drop the green dots in a big pile you can run through and fill your upgrade meter like nobody's business.
Start by getting a bigger spread of the crossbow and getting the reload that gives you two extra shots before you reload. I powered it up until I had fullisade on rapid fire and shot in all directions when the clip was empty. Get the freeze upgrades because that will be CRITICAL late in the game. Get haste
At the 5 minute mark, the dinosaur boss that charges will pen you in this electrified rectangle. Unload on him till he drops (making sure not to get greedy. Keep a path clear through the hoards of grunts). When he's dead, he drops a pickup. Choose the tome of power. It only takes away 1 max hp, but I was surprised that it only took a slot for some hp I already lost. No real sacrifice at all
By that point, the swarms get CRAZY. Just keep shoot in the direction your running. Keep a path clear. Keep moving. No more bosses. Don't do anything fancy or worry about any more upgrades at this point. Just keep moving and run out the clock
It took me SOOOO many tries and strategies, but I finally did it and let me tell you- I feel incredible! This game is so well designed and so rewarding to complete. Omigosh, 10/10 would reccomend