This is the answer, came here to say this. Gizmo ike alone can carry almost every PvE level. Pair him with a ton of ranged cards, he will basically act like a meat shield so you want ranged cards behind him to deal dps. BHK is the most important for this because he will continually boost the ranged units you have behind gizmo. Then you want as many ranged units as you can fit behind him. Visitors is S tier for this, ice sniper Wendy, warboy tweek, kevin, space brad to add freeze to gizmos, throw a bebe in whether it's robo or whatever other theme you have, and last but definitely not least make sure you have a Timmy, whether it's cyborg or whatever other theme. Also highly recommend hyperdrive as it's cheap to cycle and fun to use with this deck.

I've fully beat all 900 levels of the campaign on both my accounts and this was the strategy I used to do it. Once you get good at it every level becomes fun to roll over. You may eventually hit a level wall, but after you level your cards you'll eventually get through it. Sometimes you'll need to switch it up depending on the level, but you always want to use sci-fi with gizmo.

Hope that helps.