Oh boy.
1. Focus on Melee, since you're not going to outshoot them. Guns are for cowards anyways. Get a good load of pikemen, rodoleros and cosolete if you can afford them.
2. That said, bring a few guns. A handful of arquebusiers strewn across the landscape can draw a lot of fire while the tercio advances
3. Bring some CHEAP cavalry - low level mercs, freed units, bandits etc.
4. Make sure you can win the battle. If all goes right, winning 2:1 is possible against the IS, but don't count on it
5. When the fight starts, take cover behind terrain. Either move up towards the enemy position while always keeping distance and/or cover in between you and them, or lay in waiting behind the crest of a hill.
6a. If you moved first, go as close as you can without coming under fire. Then send a cavalry charge to disrupt them. After five seconds or so, charge with everything else. Keep the Pikemen in a formation until their cavalry has been killed or unhorsed, then split them up to mop up the gunners.
6b. If you let them come to you, their cavalry will attack first. Intercept them with either a pike block (if you're fighting uphill) or a close order pike line with the guns right behind (for downhill battles). Once the cavalry is disabled, retreat behind cover and wait for the gunners. Check on their advance occasionally. Time your attack so that you get them BEFORE they climb on top of whatever you're hiding behind - having to fight uphill is highly undesirable. Never have your pikes in a formation for this, since the enemy gunners will be spread thinly.
7. Your cheapass cavalry. Use them, as mentioned, to provoke the enemy into firing their weapons before your melee attack. This can be done before any assault, but make sure you're fairly close before giving the order. Another use is to slow down the enemy cavalry - yours won't beat them in a joust, but they will slow them down enough for your arquebusiers to hit their horses. Just remember to make your gunners retreat in time.
8. Once the melee begins, they're toast. Utter, ****ing, olive-on-pikes toast. Make sure to collect some heads for yourself.
For the Queen!