Early game is fairly easy, it might get mildly hard at the last waves, but at the last wave, when Red Boy Imp appears, if you don't have defensive plants, it's almost impossible to beat. But if you have defensive plants, it's still really hard, way harder than Dr. Zomboss fight.
Strategies
Strategy 1
High-damage lobber plants are recommended to do heavy damage and to counter the Ladder Zombie. Slowdown plants like Snow Pea and Winter Melon can also slows down zombies and buy more time to damage them further. Instant-kill plants should be saved for the boss and deal as much damage as quick as possible before it starts to shoot fireballs and destroys the entire lane of defense. However the player may be able to use Ice-shrooms.
Use Potato Mines to kill early zombies and get sun quickly, plant Repeaters for high damage and Snow Peas for slowing down zombies. Use Tall-nuts or Wall-nuts (or both of them) for defense, since the boss can deal massive damage to your plants. Lastly, use Jalapeno and Cherry Bomb for Flying Imp and most importantly Red Boy Imp, the boss.
Gallery
Level in the background of an advertisement
Walkthroughs
Plants vs. Zombies Journey To The West - Outsmart fire child 4-5 BOSS