I think it would be time to experiment with some of the less talked about moves for this one. I'm talking about squirle's water gun. Try to fully charge a water gun and wait for the projectile spammer to begin his spamming. While dodging some of his projectiles, pay close attention to his 'rang. Try to push him off the side of the map with water gun (not too hard since he's right there anyways) when his 'rang is flying around somewhere and preferably when he's shooting an arrow. What you're hopping for is his bow to remain out as he's pushed off and the 'rang-catching animation to slow him down more as he goes down. Could be worth it to short hop into water gun or any other approach. Or even a quick withdrawl and a waterfall from behind.
As for Ivysaur, bair works wonders for spacing and for stopping most projectiles. Razor leaf will stop only one, but at least it's a guaranteed stop. Aside from that, not much else for the saur.
Charizard really only has a good rocksmash to stop all but the bombs and a flamethrower to make a nice wall of protection for a while. Still the rocksmash and flamethrower can both be shothopped into (so long as you don't miss him with the thrower as well). Though a good surprise tactic would be to ride the gale boomerang back towards him with a rock or fair waiting.
Lastly, don't forget your main weapon as the trainer is to mix it up. Down B gives you loads a second or two of invincibility and a spammer may be hesitant in trying to approach you like that, since he'd have to get close and risk losing his rhythm. Because of that his having to constantly switch between different types of targets while you getting to pick the flow of the match (coupled with the fact that none of your pokemon need be stuck in a close match and tired) gives you quite a few mind game opportunities.
Mind you I've only thought of these things on the spot. And though I main and love the trainer I haven't actually tried the water gun or gliding rock-smash out yet against spammers. Still they seem to have some merit I think so if you do get to trying them out, let me know what you think. Thanks