tl;dr i liek Guise.
The way I play him, Guise is very team-oriented. Knowing the other hero decks actually helps alot, I feel, when deciding to play him or not (You should pick him anyway). Guise generally wants to go last in turn order to let other heroes play things he may want to copy. Some heroes don't offer much at all to Guise in terms of useful equipment or ongoings, like Unity or Captain Cosmic, or they may have a useless base power, like Naturalist. On this note, I haven't seen a ruling yet on whether Guise is actually allowed to have an Unlucky Pool to even use Setback's base power... oh well. In addition, Guise can function like a second Argent Adept if you copy his power or use one of his instruments, as you don't need to copy AA's songs with Uhh, Yeah to use their effects. You can be Legacy too, with Uh, Yeah and I Can Do That Too. to give +4 damage to your team (Galvanize and Inspiring Presence) for a turn. Well, 2 from you and 2 from Legacy. May as well let him think he's helping.
Guise's ongoings are all slightly situational, but all very powerful. If set up with Barbarian or other +1s, Sellout and Super Ultra Kawaii!! allow for a crazy number of damaging plays. SUK (hehe) is reaaally good at letting you play approximately a billion extra cards, especially if you have only 3 heroes in your game. But that means there are less options for you to copy, so be aware.
Otherwise, hold onto a heavy damage card like Blatant Referance until You have +1's from somewhere, such as Guise the Barbarian or Legacy. Or, if he isn't helping, just do it yourself, because You Can Do That Too! Selling Out has some really interesting mechanics behind it, because it happens at the end of the envirmonment turn. So, for example, if Obsidian Field is out, or you know it's going to come out, and you have the cards for it, you can play Selling Out on your turn and do some damage at the end of the environment, then, play Retcon as your last one-shot to stop the Villian from getting Obsidian bonus, and draw(or play) a card!
Ummm... Gritty Reboot pairs well with Legacy's Lead from the Front to absorb alot of damage and draw cards. I'm not a huge fan of Gimmicky Character. Probably becasue CC didn't turn over a construct that I needed to survive the Villian turn.
Also, you're Guise. If your hand is crap, yell at your cards. Read the italicized text. Make your turn a show.